One of South Africa's leading black journalists, whose family ranks just behind that of Nelson Mandela in esteem among antiapartheid activists here, was taken by armed men early this morning from his Soweto home, according to family members.

The South African Bureau for Information, the only government agency authorized under the country's sweeping state of emergency to release information, refused to comment on whether the journalist, Zwelakhe Sisulu, had been detained under the emergency regulations.

This article was written under South Africa's emergency regulations, which forbid journalists from disclosing the names of anyone detained by authorities under the emergency or from reporting about police activities without written government authorization.

Sisulu, whose father is serving a life sentence alongside Mandela and whose mother is copresident of the country's leading antigovernment coalition, the United Democratic Front, was seized by four whites in civilian clothing who broke through his gate and front door and refused to identify themselves, his wife, Zodwa Sisulu, said.

Two of the men wore hooded balaclava ski masks over their faces, and the license plate of their car was partially covered by a second plate with upside-down numbers, Zodwa Sisulu said.

Meanwhile, the government reported six more blacks, including a policeman, were killed during civil unrest overnight, bringing to 73 the death toll since the government imposed a nationwide state of emergency on June 12. Despite the toll, officials today maintained that unrest was decreasing due to the police crackdown.

In refusing comment on the Sisulu incident, Bureau for Information spokesman Casper Venter said, "Unfortunately, as you're well aware, we are not in a position to give any information or to comment on any detainees."

Speaking at a press briefing this afternoon after The Washington Post had telexed the bureau for comment on the incident, Venter added, "But apart from that I can tell you that we have no information in that regard, and if you have not received a telexed reply by this time it is merely because we have not been able to establish any confirmation of that."

In London, The Associated Press reported that Britain's National Union of Journalists said Friday it had received reports that four armed white vigilantes abducted Sisulu. "It begins to look at if (President P.W.) Botha's government is going down the road of death squads," said Harry Conroy, general secretary of the 33,000-member union, referring to thousands who disappeared under the former military regime in Argentina.

Sisulu's mother, Albertina, said she did not know who had taken her son and that she feared for his life. "The fact that they were masked, it means they are really dangerous people," she said.

She said her daughter-in-law had tried to demand more information from the men but had been dissuaded by her husband, who feared the men might harm the couple's two small children.

The government has refused to divulge publicly the names of those it has detained under the emergency, or their number. But security officials, chafing under allegations that activists are "disappearing," have promised in recent weeks that the next-of-kin of those detained would be informed of their arrests and whereabouts.

Nonetheless, a spokesman for the Detainees' Parents Support Committee, a human rights group here, said tonight that few of the families who have been in contact with the committee over suspected detentions had been notified by the police.

"In the vast majority of cases, they just have no idea what has happened to their relatives," said the spokesman.

Zwelakhe Sisulu has been a key figure in unifying black activists of conflicting ideologies in recent years. His father, Walter, a top official in the clandestine military wing of the African National Congress, was convicted of treason along with Mandela and six others in 1964, and has been imprisoned ever since.

His mother, Albertina, was convicted of furthering the aims of the congress in 1984 by singing freedom songs and repeating ANC slogans, and was given a two-year prison sentence. She remains free on appeal. His brother, Max, who is in exile, is a leading figure in the congress.

Sisulu was a supporter of Black Consciousness, an activist movement advocating exclusively black leadership of the antiapartheid movement that arose in opposition to the congress' multiracial approach. In recent years, he has been seen as an important bridge between the two movements because of his support for the one and his family ties to the other.

He was a founding president of the black journalists' trade union and a reporter and editor for several daily newspapers here. He served several months in detention in the late 1970s, and was "banned" between 1981 and 1983. He was a Nieman fellow at Harvard University two years ago and returned here to found The New Nation, a biweekly tabloid funded by the South African Catholic Bishops' Conference, several of whose leaders have been in detention since the emergency began.

The newspaper, which takes a political view generally in line with the United Democratic Front, has been mentioned here as a likely target for government action under the emergency. Deputy Information Minister Louis Nel told South African editors earlier this week that the government would not hesitate to use its powers to suspend publication of any newspaper it believed was violating the emergency's strict provisions.

Among the six dead reported by the government today was a municipal policeman killed along with his dog in Soweto when unknown attackers hurled two hand grenades at them. Three of the others were killed with "necklaces," burning tires placed around their necks. One badly burned corpse had seven tires wrapped around it, police said.

Because of the stringent press restrictions, it was not possible to confirm the police account of these deaths.