JOHANNESBURG, South Africa (AP) _ A former narcotics detective has been arrested in the assassinations of two prominent white activists, one in South Africa and one in Namibia, police said Thursday.

Police headquarters refused to release the name of the 31-year-old suspect, saying it confirmed his arrest only after newspapers reported it. An official statement said further arrests in the slayings were possible.

An evening news report on government-run TV said it was believed police ''are looking at the possible existence of a private assassination squad with strong right-wing links.''

The suspect, a white former police sergeant, was arrested this month in the killings of David Webster, a university lecturer and human rights activist in Johannesburg, and Anton Lubowski, a lawyer who was the only senior white official of Namibia's main independence movement, the South-West Africa People's Organization.

Both men were shot to death outside their homes; Webster on May 1 in Johannesburg and Lubowski on Sept. 12 in Windhoek, Namibia.

The suspect had not been charged and was being held for questioning under the provisions of security legislation that allows for indefinite detention without access to visitors or lawyers.

The Star, a liberal Johannesburg daily, said the suspect's father had filed an application demanding his son's release from a Johannesburg police station cell, but later withdrew the request.

Police said the suspect was discharged from the police force in 1984 after being sentenced to prison on charges of murder and theft. He was convicted of killing a suspect in one of his investigations as a narcotics officer for the West Rand, a region west of Johannesburg.

''The person has been in detention for some time,'' police said. ''The police did not wish to announce his detention because this would have jeopardized an investigation which still has to be launched.''

The national police force has come under widespread criticism for its failure to solve the Webster murder and dozens of other killings of anti- apartheid activists over the past 10 years.

The arrest coincides with a government investigation into allegations by former policemen that police death squads, acting on orders from superiors, killed numerous government opponents.

An Irish national, Donald Acheson, 51, was arrested in Namibia the day after Lubowski was killed. He had been held as an illegal immigrant, but on Nov. 6 was told he would be charged in the slaying.

Newspapers have reported that Acheson was taken to Johannesburg for interrogation by authorities who fought the court application for the ex- narcotics officer's release. Those authorities also are investigating Webster's assassination.

Also on Thursday:

- Scores of protesting black bakery workers in Pretoria reportedly were injured and 200 arrested by riot police using tear gas and clubs. Police said five officers were hurt by bread crates hurled from rooftops.

Hundreds of workers gathered at the Boerstra bakery in Pretoria to protest working conditions. Neil Coleman, a spokesman for the Congress of South African Trade Unions, said riot police attacked the workers without provocation. Police said the protesters ignored repeated warnings to disperse.

- Police in the impoverished township of Alexandra near Johannesburg used tear gas and rubber bullets to disperse 500 blacks protesting living conditions in their township. Four people reportedly were arrested.

- Six anti-apartheid activists met with African National Congress leader Nelson Mandela and later said they did not believe he would be freed from prison this year. Billy Nair, former Natal province leader of the outlawed ANC military wing, did not comment on the more prominent expectation that Mandela would be freed early next year. President F.W. de Klerk said, ''The matter is receiving attention. ... We will make an announcement when we are ready.''

- De Klerk prepared to travel to the Ivory Coast for a meeting Friday with President Felix Houphouet-Boigny to discuss mutual relations and the Angolan civil war. The two countries have no formal diplomatic relations, but extensive trade ties. Houphouet-Boigny condemns apartheid, South Africa's legal system of racial separation, but criticizes as counterproductive sanctions to force South Africa to give equal rights to its black majority.