Most of South Africa came to a standstill today as nearly 2 million black workers marked May Day with the biggest work stoppage in the country's history.

The call for a general strike to protest apartheid came mainly from the radical Congress of South African Trade Unions, which was formed only five months ago as South Africa's biggest federation of black labor unions.

One of the most crippling work stoppages was in South Africa's main industrial and mining region of the Witwatersrand, centered on Johannesburg. An independent body of specialists called the Labor Monitoring Group estimated that eight out of 10 black workers there had responded to the call for a general strike.

There was a 100 percent stoppage in the eastern Cape Province, traditional stronghold of the black activist movements and the scene of the most persistent racial unrest over the past 21 months, the monitoring group reported.

It estimated there had been a 70 percent stoppage in Natal Province but said the response had been less in western Cape Province, around Cape Town.

The main employers' organization in the country, the Association of Chambers of Commerce, agreed with these estimates. "Without doubt this is the biggest nationwide work stoppage South Africa has had," said Vincent Brett, a chamber official.

Brett added that blacks in the Witwatersrand area who wanted to work could not do so because transport services from the segregated townships had come to a standstill.

There were no bus drivers, he said, and train services from Johannesburg's big Soweto township had been disrupted by a fire -- thought to be sabotage -- that burned two passenger cars early this morning. Black taxi drivers also supported the strike, Brett said.

The action was regarded as a test of strength by the 650,000-member union congress, known by its acronym of COSATU. It also was regarded as a trial run for a three-day stoppage planned for June 16, 17 and 18 to mark the 10th anniversary of the 1976 Soweto riots.

Black activist organizations are looking to the June strike call as the central event in this year's antiapartheid campaigning.

The union congress has aligned itself publicly with the United Democratic Front, the main alliance of activist organizations in the black-majority population. Congress president Elijah Barayi also held talks recently with exiled leaders of the outlawed African National Congress.

The congress has also declared itself strongly in favor of economic sanctions, including disinvestment, as a way of pressuring the Pretoria government into abandoning apartheid and white-minority rule.

About 60,000 supporters of centrist Zulu leader Chief Gatsha Buthelezi packed a sports stadium in this Indian Ocean city today to launch a rival union opposed to the congress' call for foreign disinvestment.

At the union congress' inaugural rally last December, Barayi strongly attacked Buthelezi, accusing him of being a "puppet" of the government because he holds office as a tribal "homeland" leader within the apartheid system. This deepened an already wide rift between Buthelezi and the black nationalists.

Soon after the congress was formed, Buthelezi announced his intention to launch a rival union rather than have members of his Zulu-based Inkatha movement belong to member unions of the federation that was criticizing him.

Today's rally to inaugurate the new union, called the United Workers Union of South Africa, took place less than three miles from another stadium where 10,000 congress members where holding one of several regional rallies. A crowd of 30,000 attended a similar congress rally in Soweto.

The two Durban rallies were redolent of the resentment that has developed between Buthelezi and his rivals. The United Workers' rally opened with a black coffin bearing the words "COSATU is dead" being paraded in front of the platform where Buthelezi was seated.

Most of the Zulu chief's 70-minute speech was devoted to attacking the congress and justifying his decision to form a union in opposition to it.

At the other stadium, meanwhile, the congress' general secretary, Jay Naidoo, accused Buthelezi of exploiting Zulu tribalism and furthering Pretoria's divide-and-rule strategy.

A large contingent of armed police patrolled the city to prevent violence between supporters of the rival organizations.

Opposition to disinvestment and a cautionary approach to striking were the main themes of the United Workers' rally, where thousand of supporters wore T-shirts proclaiming the slogan, "Jobs, not hunger."

Impressive though the rally was, it gave the impression of being essentially a tribal, rather than a union, occasion. Zulu tribal regalia, songs and chants were much in evidence.

Many people interviewed in the crowd seemed to have little idea of why they were there. They had been marshalled by Inkatha regional committees and brought there in 17 specially chartered trains and scores of buses.

Mark Bennett, a labor researcher attached to Natal University, estimated that fewer than 2,000 were members of organized unions.

Labor uses May Day to mark police suppression of strikers in the Haymarket riot in Chicago in 1886.