South African authorities have launched a second-wave assault on the black labor movement as part of their emergency crackdown, detaining several major union leaders during the past few days, union sources said here today.

An independent agency, the Labor Monitoring Group, said about 920 union members were now known to be in detention, compared with 171 in a report it issued a week ago. A total of between 3,000 and 4,000 people are believed to be held.

Violence continued to escalate despite the massive security action, with nine more unrest-related deaths and another bomb explosion in a shopping center reported today.

The government also disclosed for the first time the identity of a detainee, confirming that Zwelakhe Sisulu, a black newspaper editor, had been arrested at his home about midnight Thursday.

According to the Labor Monitoring Group, most of the union members detained belong to unions affiliated with the Congress of South African Trade Unions (Cosatu), the country's largest union federation, which claims nearly half a million members.

The single most seriously affected union is the 60,000-member Metal and Allied Workers Union. It is regarded by observers here as one of the most effective unions in the relatively young black labor movement, which won the legal right to form unions only seven years ago.

The names of some of the top-ranking union leaders who have been detained are known in press circles here, but reporters may not disclose them under the stringent restrictions imposed by the Pretoria government.

Reports reaching Washington said that Elijah Barayi, president of Cosatu, and Moses Mayekiso, general secretary of the Metal and Allied Workers Union, had been detained.

Barayi was detained last night at his home in Soweto. Mayekiso, who is also a community leader in the troubled black township of Alexandra, north of Johannesburg, was arrested as he landed at Johannesburg airport Saturday morning on his return from a trip to Europe, the reports said.

The detentions are expected to trigger a wave of retaliatory strikes by black workers, further disrupting businesses that are already feeling the effects of protest action.

Since the emergency was declared on June 12, there have been more than 100 strikes in major chain stores in protest against the detention of union leaders, causing a delegation of top businessmen to appeal to the government to stop disrupting labor relations.

The Labor Monitoring Group said in its report today that 15 of the first wave of 171 unionists detained had been released after two weeks, but since then a second group of 767 had been taken.

This second wave included 737 union members taken during protest strikes at factories, together with 27 union leaders.

There are now 183 union officials and shop stewards in detention, the monitoring group said.

The official Bureau for Information said nine more blacks died in unrest-related incidents during the past 24 hours, bringing to 81 the number of deaths since the emergency was declared 16 days ago.

Four of today's casualties were said by the bureau to be guerrillas of the underground African National Congress who died in a skirmish with police at a roadblock near the Botswana border.

A policeman was hospitalized with a shrapnel wound caused by several hand grenades thrown by the guerrillas, the bureau statement said.

There were five other deaths in unrest incidents in the black townships. Two black men were stoned to death in a Soweto street, another was shot by police during a gasoline bomb attack on a police vehicle in the western Cape Province town of Worcester, and two were found burned in the northern tribal "homeland" of KwaNdebele.

Law and Order Minister Louis le Grange today issued a statement saying Sisulu, editor of a biweekly black newspaper called The New Nation, was being held by police.

The bureau had refused yesterday to confirm Sisulu's detention after his wife, Zodwa, told reporters that four armed white men, two of them wearing masks, had broken into their Soweto home at midnight and taken her husband away in a car with obscured license plates.

This led to reports in the local press speculating that Sisulu, a member of a distinguished black political family and one of the most respected black journalists here, may have been kidnaped by white vigilantes, possibly with the intention of killing him.

As concern about his safety mounted today, le Grange finally issued a statement saying that Sisulu had been detained but denying that the men who took him away had worn masks.