In the days after World War II, black South Africans living in urban townships looked with admiration on the big Detroit monsters that affluent motorists drove; they dressed flashily in what they imagined was a "Chicago style," affected an American accent and regarded Joe Louis and Louis Armstrong as the symbols of black liberation.

Today, young black radicals in those townships denounce all American political figures, even those as sympathetic to black liberation as Sen. Edward M. Kennedy (D-Mass.) or the Rev. Jesse Jackson. They link global racism to U.S. capitalism, and say the only American they are interested in listening to is Louis Farrakhan, leader of the militant black Nation of Islam.

The rise of campaigns of anticapitalism and anti-Americanism in the black townships and the decline of the American image have been underway for years. But the decline seems to have turned into a plunge since the Reagan administration came to office and began its policy of quiet diplomacy with Pretoria called "constructive engagement."

That this movement has deepened is indicated by the fact that the radicals who demonstrated against Kennedy during his South African tour this month say they may do the same to Jackson if he comes here, or any other American politician.

But they invited Farrakhan to be the keynote speaker at their convention in December. He did not come, but he sent a taped message of support that was played.

The difference, explained Imran Moosa, publicity secretary of a small, Soweto-based black consciousness party called the Azanian People's Organization (Azania is the black nationalist name for South Africa), is that Jackson aligned himself with the American "political establishment" by seeking the Democratic Party nomination, whereas the head of the Nation of Islam is seen as free of any such taint.

"Still, we cannot condemn Jackson outright," said Moosa. "He has done some things for black Americans. If he comes here . . . on a fact-finding visit, we won't mind. But if he comes here as a political messiah, then we'll demonstrate against him, too."

To the members of Azapo, as the Azanian party is known, and adherents of other groups in South Africa's militant black consciousness movement -- which together have about 600,000 members -- Kennedy was an unacceptable visitor both because he represents the U.S. "political establishment," which they regard as supportive of their country's segregationist system called apartheid, and because they feel he was grandstanding during his tour and using oppressed blacks for his own political advancement.

The hostility toward the American system stems from a belief that the United States symbolizes "racism and capitalism," which the black movement considers the core elements of apartheid.

In Soweto last Sunday, where Azapo demonstrators forced cancellation of a farewell speech by Kennedy, they handed out pamphlets that proclaimed: "Western Christian civilization, championed by the United States and its surrogate regime, apartheid South Africa, has meant the dispossession of our people and their land, exploitation and oppression. America is, like its other partners in crime such as South Africa, Israel etc., swimming in the blood and sweat of the black oppressed."

The young militants do not express hostility toward individual Americans and are pleased, if slightly cynical, about the current demonstrations outside the South African Embassy in Washington.

"But we reject giving red-carpet treatment to any member of the American political establishment," said Azapo's president, Ishmael Mkhabela, 33, the oldest member of the Central Committee elected at the party's convention last month.

Black consciousness is a minority movement in black South African politics, rating well behind the underground African National Congress and a tribally based movement led by the moderate Zulu chief Gatsha Buthelezi in opinion polls in the ghetto communities.

It has a checkered history, and there are many ironies about its current attempts to appear more truly socialist than the African National Congress, which has past links with the country's outlawed Communist Party and the Soviet Union, but few political observers doubt that it has a popular theme in the black community with its campaign of anticapitalism and anti-Americanism.

Functions at the U.S. Embassy in Pretoria, which were oases of multiracial socializing in the 1960s when the embassy became the first to break with South African convention by including blacks in its guest list, are predominantly white occasions again as many blacks decline the invitations to show their disapproval of the Reagan administration's policy.

" 'Constructive engagement' has done enormous damage to the image of the United States in the black community," said the Rev. C. F. Beyers Naude, the country's best known white Afrikaner dissident, who will become general secretary of the South African Council of Churches next month and has extensive contacts in the black community.

"Azapo does not have widespread support, but it is riding a popular wave with its anti-American campaign," Beyers Naude said.

The African National Congress and other black political groups that had favored building a nonracial society and had cooperated with white liberals were driven underground by strong government repression in the 1960s.

In 1968, a young black medical student named Steven Biko, reacting against the role of liberal white students as spokesmen for the black cause, formed an exclusively black students' organization infused with ideas of black consciousness then reaching South Africa from the United States and black Africa.

The black consciousness movement expanded rapidly, applying a race rather than a class analysis to the South African political situation. It did not, like the African National Congress, emphasize socialism in its platform.

The surge of political consciousness that Biko's movement inspired led to the Soweto uprising of 1976, in which more than 600 young blacks were killed in clashes with the police. The government reacted by banning the black consciousness organizations. The following year, Biko died in police custody.

Azapo was formed in 1978 to keep alive the political tradition, but with most of Biko's lieutenants imprisoned or in exile it made little impact.

From this low point the black consciousness movement has made a comeback during the past three years, and at the same time has undergone something of a transformation.

When a group of civic organizations and labor unions that broadly adhere to principles of the African National Congress formed a large alliance called the United Democratic Front in 1982, Azapo leaders convened a smaller alliance of black consciousness organizations called the National Forum, which now claims 600,000 members compared with the front's 2.5 million.

The forum, in addition to declaring "racism and capitalism" the core elements of apartheid, committed itself to a program of socialism.

Kennedy came at the invitation of Anglican Bishop Desmond Tutu, last year's winner of the Nobel Peace Prize, and the Rev. Allan Boesak, the president of the World Alliance of Reformed Churches. Both are closely identified with the Democratic Front.

The black consciousness leaders saw this as an opportunity to dramatize their new emphasis on socialism, and to project themselves as more militant and anticapitalist than their Democratic Front rivals, despite the front's aura as a surrogate of the underground congress with its guerrilla army and leaders in prison.

"The Kennedy visit was a watershed because it illustrated the difference between the two organizations," Neville Alexander, a socialist intellectual who served 10 years on Robben Island, a prison for political offenders, said in an interview last week. Alexander is largely responsible for the ideological change in Azapo, incorporating a class analysis into its policy, which had been based entirely on race.

"The invitation to such a representative of capitalism is precisely the kind of compromise which we believe the UDF is inclined to make," Alexander said. "It exposed the fact that they have placed middle-class leaders in charge of their organization. Whatever Kennedy may be in his own country, the idea that he could be a mediator in the race struggle here is, I would say, grotesque."

Would all Americans be received with the same hostility?

"Not necessarily," said Saths Cooper, 34, an Azapo leader who also served a term on Robben Island and now is a clinical psychologist. "Each one must be viewed in the context of what he represents in the United States, and of the kind of visit he makes here.

"Kennedy represents the American political system. He came here to boost his prospects for running for president," Cooper said. "And the manner in which he went around, like a great white god coming to offer hope and salvation, is just the kind of WASP arrogance that makes blacks here reject anything that smacks of Uncle Sam."

According to Mkhabela, who became president of Azapo a month ago in a rotation intended as a safeguard against government action, Azapo is not pro-Soviet.

"In principle we are against imperialism of any nature," he said. "But one has to admit that the eastern countries, in contrast with the West, have been seen to contribute to people's struggles against colonialism all over the world."