JOHANNESBURG, JAN. 20 -- A South African supreme court judge has reopened the controversy about police "death squads," which allegedly were involved in assassinations of anti-apartheid activists throughout the last decade, by finding that the testimony of a dissident police captain about their activities was true.

In a case with enormous ramifications for both press freedom and the government, Justice Johan Kriegler ruled Friday that the allegations of former police Capt. Dick Coetzee about involvement of the police, including himself, in assassinations of political activists here and abroad were believable.

Given his public confessions that forced him into exile abroad, it was "unthinkable" that Coetzee would have lied about his involvement in the murder and kidnapping of political activists. "Why would he declare himself guilty of such heinous crimes if it had not been the truth?" the judge asked.

The judge also found that police Lt. Gen. Lothar Neethling, who had brought a libel suit against two weekly newspapers that in late 1989 exposed his alleged role in death-squad activities, had repeatedly lied not only to him but also to an earlier government-appointed commission led by Justice Louis Harms.

Justice Kriegler's findings in favor of the two publications, the Weekly Mail and Vrye Weekblad, were immediately hailed by anti-apartheid and human-rights groups as major victories in their struggle to bring to light the activities of secret police and military squads believed responsible for more than 100 assassinations of anti-apartheid activists during the 1980s.

Spokesmen for the groups said Kriegler's findings clearly placed in serious doubt the Harms Commission report of last November exonerating the police of any involvement in the political killings.

They demanded that government and police officials who had denied the existence of, or their links to, the death squads should now be charged with perjury or murder. Vrye Weekblad editor, Max du Preez, predicted that the ruling would advance press freedom in South Africa.

The biggest loser in the case was Neethling, chief of the South African police's forensic department. He had sued for $600,000 in damages against the two weeklies for their allegations that he had supplied poisons to Coetzee for use in the assassination of anti-apartheid activists. The judge has ordered Neethling to pay the two weeklies' legal expenses, which du Preez estimated in the case of Vrye Weekblad at about $400,000. But Neethling said he would appeal.

Justice Kriegler said he had come to the "sad conclusion" that Neethling had lied and misled both himself and the Harms Commission with his denials of having ever known or met Coetzee. Kriegler said that when he re-read Neethling's testimony before both his court and the Harms Commission, "it hit him like a thunderbolt" that the general had played "a dirty trick in an open court" to hide his contacts with Coetzee. The testimony of Coetzee, on the other hand, had been consistent and believable, Kriegler concluded.