The earnest young man in a yellow T-shirt that proclaimed his membership in a black political movement paused for a moment from the singing, chanting ritual of the big funeral rally to answer his questioner. "Nelson Mandela is my leader," he said in tones of reverence. "He is a great man. He is going to lead our people to freedom."

He could not have been more than 16, which meant he had never seen the subject of his admiration nor heard him speak. Nelson Mandela, 67, the acknowledged leader of South Africa's main black nationalist movement, the outlawed African National Congress, was imprisoned for life 23 years ago for sabotage.

Since then, only Mandela's family has been allowed to visit him regularly and only an occasional second-hand report from him has seeped through the controls and gag laws that enclose his cell.

Yet, instead of fading from memory as the government hoped when it sent him to a windswept prison off Cape Town in August 1962, Mandela's fame has grown with the years into a legend that has assumed an almost messianic importance to many of the 21 million blacks who live under the apartheid system of white-minority rule.

The swelling demand for his release could lead to a major confrontation Wednesday as Allan Boesak, a pastor who plays a prominent role in the black activist movement, prepares to lead 20,000 demonstrators in a march to Cape Town's Pollsmoor Prison, where Mandela now is held, in defiance of a warning by Law and Order Minister Louis le Grange that the march is illegal and that police will stop it.

It is not only the militant youth who respond to the Mandela legend. Even black political opponents, including conservatives who work within the apartheid administration and are despised by the ANC, acknowledge it and join ritually in the calls for Mandela's release. To do otherwise would be seen as an act of sacrilege in the black community.

An opinion poll published in a Johannesburg newspaper today showed that more than 90 percent of the country's blacks want Mandela released unconditionally. "He has become the symbol of our people," said Nobel Peace laureate Bishop Desmond Tutu. "His imprisonment represents their oppression. His self-sacrifice is what we would all like to be in resisting that oppression, and his release has come to symbolize the liberation they are longing for.

"It may seem almost childish, but the faith in him is so complete that somehow I think people believe that if he were to come out, things would be all right."

Lucas Mangope, president of the nominally independent tribal "homeland" of Bophuthatswana, said, "Freeing Mandela unconditionally would make it possible for the government to unban the ANC and start negotiating with it."

Chief Gatsha Buthelezi, the influential Zulu leader who also heads a homeland administration, has made similar calls for Mandela's release, but his declared hostility toward the African National Congress suggests that he has greater respect for the personal symbol than for Mandela's political policies.

Mandela's stature among his own people and his growing reputation abroad have become major political problems for the Pretoria government, which now finds itself involved in a sort of shadow negotiation with its prisoner over the conditions under which he might accept his own release and over the policies of his outlawed organization.

For seven months, a kind of dialogue has been going on through the media between Mandela and President Pieter W. Botha as first one, then the other, has defined his position -- Mandela speaking through members of his family and other occasional visitors and the president replying in major public pronouncements. This spectacle of the long-term political prisoner forcing his chief jailer to respond to him, and thereby increasing the international pressures on the white government, has added to Mandela's reputation.

In a way, said Tom Lodge, a specialist in black politics at Johannesburg's Witwatersrand University, the Botha government has become Mandela's prisoner, trapped in the choices it faces over what to do with the growing legend in its cell.

"On the one hand, it would like to demythologize him by releasing him," said Lodge. "It is also worried that he may die in prison, which would cause a massive black reaction.

"On the other hand, it is worried about the impact he would have on the black population if he were released to become politically active again. And Mandela is not making it easy for them. He is playing it tough, refusing to accept any conditions and making it clear that he will not renounce the ANC or any of its policies."

Last December, Mandela rejected a government offer to release him into the Transkei tribal homeland, which is run by a relative, Chief Kaiser Matanzima. Mandela made it clear he would accept no restrictions, nor have anything to do with what he regards as puppet states established under the apartheid system.

In February, Botha offered to free him if he would renounce the ANC's commitment to guerrilla struggle. Again he rejected the offer out of hand.

"Only free men can negotiate. Prisoners cannot enter into contracts," he said in a statement read for him at a rally in Soweto by his daughter, Zindziswa, 23, adding that "I cannot and will not give any undertaking at a time when I, and you the people, are not free."

His wife, Winnie Mandela, 52, who has herself been restricted during the 23 years, said in an interview yesterday: "If they were to release him unconditionally, he would come right back here to Soweto and pick up where he left off when he was sent to prison. He would completely disregard the banning of the ANC and all the other laws imposed on him by a government he does not recognize.

"I do believe he would be back in prison within hours."

By the accounts of those who knew him before, and the few who have seen him in prison, the figure behind the legend is a man of considerable presence -- tall, now gray-haired, vigorously healthy, with a dignified bearing that visitors describe as being incongruous in the prison surroundings.

"Throughout our meeting I felt that I was in the presence not of a guerrilla fighter or racial ideologue, but of a head of state," wrote Samuel Dash, former chief counsel to the Senate Watergate Committee and now a director of the International League of Human Rights, who saw him last January.

Helen Suzman, a white liberal South African member of Parliament who has been his most frequent visitor outside the family, described him as "quite indomitable," with a warm, outgoing personality. "There is nothing withdrawn or depressed about him."

Even the white prison warders show him respect, according to Neville Alexander, who spent 10 years with Mandela in a small "leadership group" of political prisoners who were separated from the others on Robben Island, where Mandela was held until he was moved with four others to Pollsmoor Prison, outside Cape Town, three years ago.

Alexander, a Colored (mixed race) intellectual strongly opposed to the ANC politically, recalled "the quite outstanding force of Mandela's personality . . . . He effortlessly attracts people and for most of them inspires total veneration."

Nelson Rolihlahla Mandela was born on July 18, 1918, a prince of the royal house of Tembuland, which now falls within Transkei. Some think his royal upbringing freed him at an early age from the emotional and intellectual shackles of second-class citizenship and helped build the assured manner he later displayed.

As a student at the black university of Fort Hare he is said to have become convinced that the prospect of tribal rule was not to his liking. He became involved in African nationalist politics and was expelled for helping to organize a student strike.

Mandela distanced himself still further from the tribal system when he fled from an attempt by the Tembu chiefs to arrange a marriage for him. He went to Johannesburg, where for a time he worked as a policeman for a gold-mining company, patrolling the huge compounds where the black miners are housed.

Mandela eventually became apprenticed to a firm of white lawyers and later established his own law firm in partnership with Oliver Tambo, a lifelong friend who is now the president of the ANC in exile.

Tambo and Mandela rose rapidly together through the ranks of the ANC, the oldest black nationalist movement on the African continent. Founded in 1912, the ANC at that stage was still a moderate, almost deferential organization whose leaders petitioned the white government politely for a better deal but never dreamed of seeking majority rule.

When a white Afrikaner nationalist government came to power in 1948 and tightened segregation with its apartheid ideology, the two young lawyers formed a militant wing called the Youth League and soon imposed its views on the old organization. "Mandela showed at an early stage that he had a good analytical mind and a sense of long-term strategy that not many of his generation displayed," said Lodge.

Mandela and Tambo foresaw that the ANC might have to operate underground, and in the mid-1950s Mandela devised a program, called the M-Plan, for establishing cells on every street in the black townships that would enable the organization to continue functioning if its national and branch leaders were imprisoned.

"If the ANC had managed to implement the M-Plan properly, the government would have found it much more difficult to crush its organizational structure after banning it in 1960," Lodge noted.

As the government began closing in on the increasingly militant organization, Mandela was first given a suspended sentence for organizing a campaign of civil disobedience, then silenced and restricted in his movements with a banning order under a catch-all security law called the Suppression of Communism Act. Put on trial for treason, he and 155 codefendants were acquitted after four years.

In the international outcry that followed the police shooting of 69 passive resisters in the black township of Sharpeville in 1960, the government outlawed the ANC. The organization, which until then had waged Gandhi-style passive resistance campaigns to try to pressure the government into ending apartheid and calling an all-race convention to write a new constitution, decided that peaceful methods were no longer possible.

The ANC has long had an alliance with the small, predominantly white South African Communist Party, which was outlawed in 1950 and today receives arms and other aid for its guerrilla forces from the Soviet Union.

There are several known Communists on the ANC national executive committee, the most conspicuous being Joe Slovo, a white who once was a leading lawyer in Johannesburg. But both Mandela and Tambo have denied that the ANC is itself Communist, as Pretoria claims.

Samuel Dash wrote after seeing Mandela that the imprisoned leader dismissed charges that the ANC was controlled from Moscow and "emphasized the congress' independence and discipline, comparing its Communist members to radicals in Britain and other western democracies."

Announcing the 1960 decision to switch to guerrilla struggle in what was to be his last public speech, Mandela, then a provincial president of the ANC, went underground and formed a military wing called the Spear of the Nation.

As its first commander, Mandela slipped abroad to arrange training facilities in Algeria and undergo a brief training course himself. He returned to South Africa to a daredevil underground existence, tales of which have added greatly to his romantic image among blacks.

Dubbed the "Black Pimpernel" by the South African newspapers, Mandela dodged the police for 18 months, issuing statements and giving clandestine interviews. His biographer, Mary Benson, recalls how an amused Mandela drove her around Johannesburg, playing the role of her black chauffeur.

Secret meetings were arranged with his young wife, whom he had married only months before he went underground. Winnie Mandela recalls one occasion when a black ambulance driver took her, posing as a woman in labor and with his siren wailing, through a police barrier to a house where Nelson was hiding.

Eventually Mandela was caught and sentenced to five years' imprisonment for incitement and for leaving the country illegally.

A year later he was back in court, on charges of sabotage, this time as the main defendant in a trial featuring the entire high command of the Spear of the Nation: blacks, Asians and whites caught on a small farm near Johannesburg. All were sentenced to life imprisonment.

The trial was probably the most important single event contributing to the Mandela image. With crowds attending court each day, he conducted himself, in the words of a defense lawyer, "in a manner that was almost regal," ending with a Socratic address to the court that has become part of the litany of the black resistance.

Mandela spoke of his ideal of a democratic and free society in which all people could live together in harmony and with equal opportunities, then declared: "It is an ideal which I hope to live for and to achieve. But if needs be, it is an ideal for which I am prepared to die."

His long incarceration has honed that message. There has been no act of compromise or misjudgment to tarnish the image of self-sacrifice. His invisibility has added to the mystique.

There, for a time, the story seemed to end. The ANC, its leaders jailed and its organization broken by extensive grass-roots arrests, seemed no longer to exist as a viable political movement, above or below ground. The Spear's guerrillas made occasional bomb attacks and were usually caught almost immediately.

Mandela and his colleagues toiled in gravel pits on Robben Island through wet winters, lashed by winds blowing from Antarctica. Life imprisonment for political prisoners in South Africa means life, with no parole, and Mandela's sentence was life plus five years.

From the moment Mandela went to prison, however, the ensign of defiance was taken over by his wife.

Nomzamo Winnie Madikizela was only 20 when she left a remote rural district in the Transkei and traveled to Johannesburg to become the first black woman in the country to qualify as a medical social worker. She knew little of politics but when she met Mandela, then 35, she says she immediately fell in love.

It was not much of a preparation for the life to follow, and Winnie Mandela admits today that she had little idea of what awaited her.

On the day Mandela was sentenced to life imprisonment, Mrs. Mandela emerged smiling on the steps of the provincial Supreme Court building in Pretoria, the capital and Afrikaner heartland, wearing a full-length ocher robe of her Tembu tribe with a matching turban. The black crowd in the square below cheered her.

She has continued her role of defiance, although she has lived under restriction orders limiting her movements and prohibiting her from being in the company of more than one person at a time.

Eight years ago she was banished to the remote country town of Brandfort in Orange Free State Province, 250 miles south of Johannesburg. She is currently defying this restriction and living in Johannesburg following the firebombing of her home and clinic by unknown assailants 12 days ago.

She has been subjected to frequent police raids, has been charged almost every year with breaching her restriction orders and has been imprisoned several times.

"Winnie Mandela has made a major contribution to Nelson's image," said Tutu. "They have become a symbolic couple with their incredible strength and refusal to be broken."

While their political rhetoric is strident and unyielding, the two Mandelas reveal a different side of themselves when speaking of their own relationship. The language of Mandela's letters to his wife in particular would almost seem more appropriate to a Victorian novelist than to a man regarded as a hardened revolutionary.

"Your love and patient support have given me so much strength and support," he wrote in a recent letter, "yet there have been moments when that love and happiness, that trust and hope, have turned into pure agony, when conscience and a sense of guilt have ravaged every part of my being, when I have wondered whether any kind of commitment can ever be sufficient excuse for abandoning a young and inexperienced woman in a pitiless desert, literally throwing her into the hands of highwaymen; a wonderful woman without her pillar and support at times of need."

Winnie Mandela speaks in longing tones of what life might have been. Their silver wedding anniversary was last year, but she estimates that they have spent a total of only four months together, in broken spells.

"It has been a great marriage," she said with a slow smile, "the marriage of the century. But I still feel like that young girl of 20 when I first met him, still looking forward to being married to him and living with him and our family."

It was Winnie Mandela who last Wednesday conveyed the latest message from her husband in the strange dialogue between him and Botha, a hard-line statement that he would refuse to join a constitutional convention with the government if it released him.

"The only thing that is left to be discussed by the people of this country and the ruling Afrikaners is the handing over of power to the majority," she quoted him as saying.

The statement dismayed white liberals and some members of what is known as the "enlightened" wing of Botha's National Party who see Mandela as the only black leader with across-the-board support in the fragmented black community and say the only way to avoid the looming prospect of an all-out racial confrontation is for the government to strike a deal with him.

In fact, Mandela restated the ANC's decision that he had spelled out before going underground nearly 25 years ago, abandoning the quest for a constitutional convention and committing itself to trying to overthrow white-minority rule by violent struggle. The congress formally endorsed that decision at a conference held in exile in Morogoro, Tanzania, in 1969.

"I think what Mandela was doing in the light of all the speculation was to make it clear that if he is released he will come out as a member of the ANC who is still committed to its policies and will not turn against the leadership in exile," Lodge said.

Lodge also suggested that Mandela was responding to a hardening of black attitudes during the current wave of racial unrest, in which more than 600 blacks have died, and following the government's declaration of a state of emergency July 21 that has led to more than 2,200 arrests in the black ghettos.

Winnie Mandela hinted that her husband's refusal to negotiate is not immutable. "If the government abandons apartheid, lifts the ban on the ANC, releases all political prisoners and allows the exiles to return, then Nelson and the other ANC leaders would be prepared to sit down and talk," she said yesterday.

Botha seems a long way from considering that. He has ruled out the one-person, one-vote formula the ANC demands and made it clear that the only changes he is prepared to consider allow for little more than black participation and consultation in the current political system.