Zimbabwean Prime Minister Robert Mugabe last week met with officials of South Africa's most active guerrilla organization, the African National Congress, to discuss what facilities the movement will have in neighboring Zimbabwe.

The meeting was in line with Mugabe's pledge to give moral and diplomatic, although not military, support to the Congress and another South African guerrilla group.

Although the Congress' activities in Zimbabwe will inevitably be defined by the requirements of Mugabe's trade and economic ties with Pretoria and by the readiness of South Africa to strike militarily against any concentration of guerrillas, the possibility that the Congress will open offices in Salisbury means an opportunity to establish itself closer to home, where it is outlawed.

From nearby Zimbabwe the Congress' propaganda efforts, even if lowkey, are likely to give emphasis to growing support for South Africa's oldest and most militant black organization among people inside this country.

Both black and white opponents of the government's racial policies say that despite a 20-year ban, the Congress has emerged from stagnation to become the strongest and most effective force among those fighting for majority rule in this country. Numerous incidents within the past year bear out that perception.

The fires set by guerrillas in June at South Africa's strategic oil manufacturing complex brought it home most dramatically. That sabotage, causing more than $7 million in losses, came after a string of assassinations of black police officers and informers as well as attacks on police stations and railway lines.

Perhaps most unnerving to whites was the drama in a Pretoria bank in January when two of the 25 white hostages held by three Congress gunmen for seven hours were killed as police stormed the bank. Nine blacks are on trial in Pretoria Supreme Court on the capital offenses of high treason and murder for helping to plan that hostage siege and another attack on a police station late last year. In the past year, 19 other trials have been held involving 57 people with Congress affiliation, according to newspaper accounts.

Although these activities make the Congress the main foe of police in the low-level insurgency rumbling across South Africa, it is a long way from exerting the kind of pressure on the white government and its tough military establishment that Zimbabwe's guerrilla movements succeeded in doing in their country.

In a recent interview in Zimbabwe, Congress President Oliver Tambo, 63, said he is not worried about that.

"Our approach is, we want to reach certain goals and put everything into achieving them and we must be prepared for it to take a long time."

"But Mozambique became independent much before anyone thought it would, and if you'd asked Mugabe years ago how long he thought it [TEXT OMMITTED FROM SOURCE] would have said," added Tambo, who has lived in exile for 20 years.

The former lawyer said the first prerequisite for talks between the Congress and Pretoria is the release of all political prisoners.

"If they were out, there would be a whole lot of other things to talk about, but we have not reached the stage to present any other preconditions until their release," Tambo said in a telephone interview last week. He said the Congress would open offices in Salisbury "sometime in the future."

Prominent among the prisoners is former Congress president Nelson Mandela, who is serving a life sentence in the political prison of Robben Island for a conviction of sabotage. More than 70,000 people signed petitions this year for the government to commute his sentence, another indication of the Congress' popular appeal.

Although the Congress' activities have made it a terrorist organization in the minds of most whites here, a white South African security police officer testifying last week at the Pretoria trial likened it to Prime Minister Pieter W. Botha's ruling party.

"The ANC is really black nationalism," said Col. H. Stadler. "Its history is not dissimilar to that of any nationalist movement, including the National Party," he said, adding that the Congress turned to violence only when it decided nonviolence would not bring change.

From its founding in 1912 by a group of politically active blacks, the Congress was the center of black protest against the government's racial policies. But by 1955 when it joined four other antiapartheid groups in adopting the "Freedom Charter" that called for one man, one vote in a unitary state; equal rights in a nonracial society, and a redistribution of the country's wealth, it had won support from members of all racial groups in South Africa.

The Congress decided to initiate a guerrilla war against the government after it was banned in 1960. Mandela, now 62, was the moving force behind the formation of its military wing, Umkhonto We Sizwe, or "Spear of the Nation," until his capture in 1961.

During the 1960s and 1970s, claims by its leadership in foreign capitals to be a guerrilla movement were not backed up by much action at home. A new generation involved in the black consciousness movement of Steve Biko criticized their exiled Congress leadership for its inertia and soft living abroad.

The roots of the Congress' present revival go back four years to the rebellion of this generation in the riots of 1976, after which thousands of youths fled the country to escape detention at home. Former police minister Jimmy Kruger crushed the black consciousness movement by banning 18 black political organizations and detaining 50 black leaders in October 1977.

With an already existing underground cell structure and military training facilities in Angola, Libya and the Soviet Union, the Congress was the logical haven for these youths, now convinced that peaceful, overt protest was impossible. Police Minister Louis le Grange recently said that 6,000 youths have left the country since the 1976 uprising and a police spokesman said about 4,000 of them are getting military training.

The Congress has been aided by the failed attempts of the black consciousness movement to resuscitate itself organizationally inside South Africa. Abroad, the movement's exiled leadership cadres have been split on whether to form an external movement or join the Congress. According to most reports reaching here, the majority have opted to give tacit if not active support to the Congress.

Besides giving the Congress willing hearts and feet to carry out its guerrilla activities, the injection of this "new blood" since 1976 with hardened attitudes against working with whites and a commitment to the socialist ideas, is expected by most observers to put its own imprint on the Congress' internal structure and character in years to come.

Another important influence on the Congress' future direction and philosophy will be the source of its funding and weaponry. According to most observers, the main source at the moment is the Soviet Union. It is a common perception among both blacks and whites in South Africa, and a claim of South African government intelligence, that members of South Africa's outlawed Communist Party, many of whom are whites, wield an inordinate amount of power within the Congress because of their control of its finances and guerrilla training facilites.

Though some Congress leaders have good personal relations with U.S. government officials, there are no official ties between the Congress and Washington.

Finally, although a decline of another rebel group, Pan Africanist Congress has removed its main rival abroad, the African National Congress faces a potentially powerful competitor inside the country in Zulu Chief Gatsha Buthelezi.

A member of the Congress when he was a student, Buthelezi has a large following among the country's 5 million Zulus and is seeking to extend his power base into a national one. "We are a liberation movement, too," one of his top aides said recently.

Recent attempts by Buthelezi and the Congress to form a working alliance have so far foundered on mutual suspicions and some observers fear that if they do not succeed, the energies of both groups will be sapped by a confrontation with each other rather than with the white government they say is their opponent.