DURBAN, SOUTH AFRICA, JULY 2 -- Hundreds of thousands of black workers heeded a call today by the African National Congress for a one-day strike aimed at isolating the rival black nationalist movement of Chief Mangosuthu Buthelezi and pressing the government into stripping him of his powers.

It was the first time that the ANC, South Africa's most popular black nationalist group, has resorted to national action in an attempt to break the power of another black organization.

The tactic was denounced by Buthelezi's Inkatha organization, which is battling the ANC and its supporters in Natal province. It was also opposed by other black groups, including the Pan-Africanist Congress and its allied labor union, NACTU, as well as the Azanian People's Organization.

In addition, the white business community was concerned by the unprecedented resort to a strike by one black group against another, with one industrial association estimating the nationwide cost in lost production at close to $300 million.

Support for the strike varied widely across South Africa, with the highest rate of absenteeism -- around 90 percent -- reported in the Port Elizabeth area and the lowest -- 10 percent or less -- in Cape Town and the Orange Free State, according to police, business and ANC estimates.

In the country's industrial and mining heartland around Johannesburg, 80 to 90 percent of the black labor force stayed at home, while here in Durban, adjacent to Buthelezi's KwaZulu homeland, 67 percent of the work force joined the protest strike, according to estimates provided by the pro-ANC Congress of South African Trade Union.

The ANC and its allies had trained 3,000 marshals to make sure its supporters adhered to a "code of conduct" drawn up by the South African Council of Churches aimed at avoiding intimidation and violence. In addition, the police and army were out in force in the nation's black townships to ensure that those willing to go to work could do so.

By the end of the day, the strike seemed to have gone off relatively peacefully, with only a few incidents of violence reported. Several strike breakers expressed concern, however, about their safety when they returned home.

The ANC and its supporters have been at odds with Buthelezi's Inkatha for years. Inkatha is a political movement based in Natal province's KwaZulu tribal homeland -- one of 10 nominally self-governing areas for South Africa's main black ethnic groups -- and it has traditionally worked to change the apartheid system of racial discrimination from within. The ANC, a more ethnically diverse movement, opposed apartheid from outside the system -- and with weapons -- from the time it was banned in the early 1960s.

While there has been fighting among blacks in different areas of South Africa, the conflict in Natal has left the region ravaged, with some 3,500 killed there since January 1987. Concern over the conflict has heightened since ANC leader Nelson Mandela was released in February after 27 1/2 years in prison.

The ANC expressed delight with the turnout for today' strike, saying it showed that black opponents "do not enjoy the support on the ground."

The ANC also said the strike proved that the majority of the black community backed its demands for the government to end the conflict in Natal by stripping Buthelezi -- who is chief minister of KwaZulu -- of his police powers and arresting his "warlords."

But a former close Buthelezi aide and peace mediator, Oscar Dhlomo, said the lack of unity among black groups toward the strike would detract from its purpose of isolating Buthelezi, and he criticized the ANC for failing to consult beforehand with the Pan-Africanist Congress and the Azanian People's Organization.

Dhlomo expressed doubts that the week-long series of protests that the ANC and its allies are planning to turn the Natal conflict into a priority national issue for President Frederik W. de Klerk would do much to end the violence here.

"It's not helpful to blame one side, or to think that you can stop the violence by talking over the head of Buthelezi and Inkatha to de Klerk," he said, adding that "the violence in Natal is already a national issue."

Dhlomo has just resigned as Inkatha's secretary general and is said to have had doubts about Buthelezi's allegedly autocratic leadership. He was Inkatha's chief negotiator in various failed efforts to find a peaceful accommodation with pro-ANC groups.

Dlomo said he thought the relative success of the strike was largely due to the ability of the ANC and its allies to persuade taxi and bus owners to stay home, thus making it impossible for workers to get from their isolated townships to the industrial areas of white South Africa.

"They prevented people from going to work by stopping their means of transportation," he said. "If there are no trains or taxis, the strike is bound to be successful."

The effectiveness of this tactic was visible today in and around the Pietermaritzburg, Natal's capital, where the fighting between supporters of Inkatha and the ANC has been so fierce that the nearby Edenvale Valley has come to be called "the killing fields" of South Africa.

Police cars, army foot patrols and armored personnel carriers filled with troops were patrolling the length of the valley early this morning to protect those wishing to go to work. A small number of Inkatha supporters from the outlying hills came to work in convoys led by police vehicles, but knots of people could be seen standing along the roadside looking in vain for a taxi to take them into the city.