A group of young black activists met in Soweto the other day to discuss a threat by Nobel laureate Bishop Desmond Tutu, made in a moment of exasperation, to leave South Africa if they did not stop killing black policemen and others regarded as collaborators in the apartheid system of segregation.

Their conclusion, according to Tutu, was: "Let him go, then we can get on with the revolution without him restraining us."

Three days later President Pieter W. Botha responded to white South Africa's perception of Tutu as a dangerous radical by turning aside his request for a meeting.

The hostility toward Tutu contained in both these views illustrates the difficulties the Nobel Peace Prize winner faces as the foremost spokesman for black South Africans in the country's present racial crisis.

Recent interview and speeches make it clear that he is the man in the middle of a gathering confrontation, pleading with one side to yield and with the other to show restraint, trying to maintain his credibility with black radicals who believe the time for talking is past, while attempting to establish a basis for negotiation with a hostile white minority government that regards any call for black rule as extremist.

It is a tight position to be in. To strengthen his hand in one direction is to weaken it in the other. Tutu is trying to breathe inspiration into the blacks as they face an awesome regime, and at the same time trying to keep their fury under control.

When he requested a meeting with Botha last week to try to defuse the crisis, observers agree that he made a small impact on the whites but lost ground among the black radicals.

When Botha snubbed him, many whites said he had been put in his place, while his credibility among the black militants rose again.

"I am the marginal man between two forces, and possibly I will be crushed," Tutu said. "But that is where God has placed me and I have accepted the vocation."

Tutu is often compared with Martin Luther King Jr., and criticism by young black radicals of his nonviolent stance is one of many similarities between them.

Tutu, like King, is a clergyman -- he is the Anglican bishop of Johannesburg -- and he operates from the same ideological base of nonviolent Christian morality that the American civil rights leader did.

He uses his ecclesiastical position in the same way to address a deeply spiritual community. He offers the same blend of moral passion and physical courage, and his speeches echo King's rhetorical cadences.

But there are major differences. Tutu has no organizational equivalent of King's Southern Christian Leadership Conference. He is a patron of the United Democratic Front and the black consciousness movement, both of which are loose alliances of community-based organizations with their own leadership. Tutu stays close to the United Democratic Front and cooperates with it, but he has no organization of his own to plan and implement a campaign of civil disobedience.

He has been criticized for being a loner, making his own decisions and relying on his charismatic personality to project his themes.

The biggest difference is that Tutu is operating in a society where the political tradition and the constitution are stacked against him.

King was able to campaign for rights guaranteed under the U.S. Constitution, Tutu pointed out, whereas in South Africa both the constitution and the law entrench segregation and isolation of blacks, who make up about 80 percent of the population, from the nation's politics.

"I think, too," said Tutu, "that nonviolent action requires a minimum moral level in the society to which appeal can be made and which will be outraged by some of the things it sees happening, and I am not sure that we have got that here."

King's strategy was to pressure local communities to make changes, while at the same time appealing to the U.S. government and the American people at large for federal action.

Tutu cannot do that and he appeals to the world community instead.

It is here that he has had his main success. Tutu has attained a level of international acclaim and diplomatic influence unequaled by any other black South African.

The Nobel award last year, and the way he seized his moment in the spotlight to dramatize his people's cause, put him on a world stage so that he now has access to heads of state in many countries.

His lobbying and frequent public appearances in the United States, where he was on sabbatical when the Nobel award was announced, served as a catalyst in mobilizing the demonstrations at the South African Embassy in Washington and gave impetus to the sanctions measures now before Congress.

Last May he met with French Prime Minister Laurent Fabius and appealed for increased pressure on South Africa. Two weeks ago France recalled its ambassador and announced an investment freeze.

This international success has done much to establish Tutu as a hero in the black townships. The Nobel Prize helped, too, demonstrating to a population haunted by its second-class status that one of them could become a world figure.

But each time Tutu makes one of his frequent appeals to the activists not to "spoil our wonderful cause" with their acts of retribution against the "sellouts," the crowd falls silent. There is a sense that he has lost another bit of his credibility.

Conversations with people in those audiences sometimes reveal an ambiguity in attitudes toward him. A young activist who came out of hiding from the security police last Thursday to help organize a funeral rally said admiringly of Tutu: "He is my leader. He speaks for me."

Asked whether he agreed with Tutu's calls for nonviolence, the young man replied: "No, I am against the bishop there. I agree with him that God is on our side, but that is not enough to win us the struggle. We must join Umkhonto we Sizwe and get an 'AK.' " This was a reference to the guerrilla wing of the underground African National Congress and the Soviet-designed AK47 rifles its guerrillas use.

C.F. Beyers Naude, a dissident Afrikaner clergyman who was banned by the government for seven years and now has succeeded Tutu as secretary general of the South African Council of Churches, where he has close contact with blacks, said Tutu's standing in the townships is still high. But, he added, with each appeal for restraint and each attempt to get into a dialogue with the government that standing diminishes among those setting the pace of the black rebellion.

Somewhere there is a point where he may cease to have meaningful influence over the activists. If he passes that point, there is no national figure outside the jail cells who could replace him.

Most whites have no sense that here is a man on a knife's edge, trying desperately to prevent the black rebellion from spilling over into unrestrained violence.

Although Tutu has his admirers among the liberals, most whites hate him. No other black South African, not even the leaders of the outlawed African congress, has been subjected to such sustained vilification by the progovernment newspapers and broadcasting services.

"I think this is because Desmond represents a challenge to white South Africa's whole self-image of Christian morality," Naude said.

"It is also because they feel threatened by his articulateness and self-confidence," Naude added. "He represents a threat to their basic concepts about race on which they have built their whole life and ideology."

In these circumstances, to play the role Tutu has cast for himself, of both activist leader and racial conciliator, would seem to call for a high degree of political adroitness. Yet Tutu lays no claim to being a politician.

"I am just a religious leader standing in for the real leaders of our people who are in jail and in exile," he said, referring to the leaders of the African congress.

This means that his actions are based on the dictates of his faith rather than on political considerations.

"I am trying to follow biblical paradigms," he said, "where Moses goes to see pharoah not once but several times, even when he is told that pharoah is going to harden his heart."

Tutu's only meeting with Botha, in 1981, was a political disaster. He was harshly criticized by black radicals and in the end came away with nothing. Now he has been snubbed. Yet he says he will keep trying to open a dialogue.

"As a church leader it's very difficult for me to say, 'Go to hell' -- to say God's grace cannot operate on P.W. Botha," he explains.

"Desmond is a prisoner of the Gospel," said one young activist.

Does this make him an ineffective political leader, as some of the black radicals are beginning to suggest?

No, said Tom Lodge, who teaches black politics at Johannesburg's Witwatersrand University. Lodge thinks Tutu has "great political instincts" and a canny intuition for the long-term effects of what he is doing.

Far from being naive, Lodge said, Tutu's efforts to stop the reprisal killings are well judged politically, because if the violent acts continue they will reduce the black campaign's international and moral support.