JOHANNESBURG, JAN. 12 -- Gunmen opened fire with automatic rifles on a crowd attending an all-night wake for an African National Congress activist in the black township of Sebokeng early today, killing 35 mourners and wounding 40.

The massacre, reportedly carried out by a local gang of car thieves, sparked harsh criticism of the South African police -- whom residents charged had been warned beforehand of a possible attack. Police denied the charge, saying ANC members had asked them to withdraw a special unit sent to provide protection for the mourners.

Law and Order Minister Adriaan Vlok called the slayings "a most shocking and horrifying deed." He assured Sebokeng residents that the police "will not rest until the killers have been tracked down."

The massacre threatened to touch off another round of violence in Sebokeng, south of here, as several thousand ANC supporters marched through the streets and set fire to houses of three persons suspected of belonging to the gang.

Political violence has periodically turned Sebokeng into a battlefield since the legalization of anti-apartheid groups by the government last February. The conflicts involved either the police against anti-apartheid activists, or ANC supporters against those of Zulu Chief Mangosuthu Buthelezi.

This time, however, the conflict reportedly involved the ANC and a local gang of car thieves and troublemakers that ANC supporters were seeking to curb, according to members of the dead activist's family.

They said the youth, Mphikeleli Christoffel Nangalembe, had been kidnapped by the gang Jan. 5 and found strangled at a township dump the next day.

His brother told reporters who visited the township today that he had provided the police with the name and address of the gang leader on the day of the kidnapping but charged that they had refused to take any action.

The family and survivors have provided the police with the names of a number of alleged gang members. Some ANC officials claimed the gang was hired by Buthelezi's Inkatha Freedom Party to carry out the attack but offered no evidence.

According to one report, one of the gang members involved in the attack had been arrested two days ago for the illegal possession of an AK-47 rifle and released on bail.

Nangalembe family members and local ANC officials gave varying accounts of the early morning attack. They said two to four cars carrying 10 to 20 men armed with AK-47 rifles arrived at the tent where 200 to 300 people were attending the wake and opened fire.

"They never said anything. They just opened fire," said a survivor who was shot twice in the leg. Twenty-seven people died immediately and at least eight others had died of injuries by tonight, according to the police and residents.

Nangalembe's father and a brother were reported to have died in the hail of bullets.