The South African security police today announced the first death of a political detainee in their custody since the leader of the country's black consciousness movement, Steven Biko, died in 1977 of injuries including damage to his brain.

The dead man is a 27-year-old white doctor, Neil Aggett, who had given up full-time medical practice to work as the secretary for a black trade union.

He was detained in a security police swoop on black unionists and their advisers Nov. 27 and had been held since Dec. 11 under an antiterrorism law providing for solitary confinement and interrogation.

Col. Hennie Muller, chief of the security police in Johannesburg, said Aggett was found hanged in his cell at security police headquarters. Aggett's colleagues in the food and canning workers' union immediately expressed doubt that he had committed suicide.

His colleagues and his sister, Jill Berger, described Aggett as an exceptionally stable and mature person whom they could not imagine becoming suicidal.

David Lewis, a close friend who is secretary of the General Workers Union, said: "He was a remarkably stable kind of guy. Either he must have been submitted to pressure beyond belief or he was killed."

Aggett's lawyer, David Dyson said the police had ordered an autopsy today without notifying the family.

When he learned of the autopsy, he telephoned the police to request that a pathologist chosen by the family be allowed to attend. The police then ordered the government doctor doing the autopsy to stop.

"He waited for the family pathologist to arrive, but the autopsy was more than half completed by the time we got him there," Dyson said.

Under South African law an autopsy must follow an unnatural death, but the authorities usually notify the family in advance to give relatives the opportunity to have their own doctor present.

The family's pathologist, Dr. Jan Botha, said tonight that tissue and blood samples had been sent for laboratory tests and he could not comment until the results were known.

Berger said her brother had been allowed to telephone her from security police headquarters the night after his detention. He had told her not to worry because "they have got nothing on me," she said.

She and her mother visited Aggett on New Year's Eve, an unusual concession by the government because detainees are usually kept incommunicado.

"He was calm and didn't seem at all twitchy," Berger said. "He told us he would probably be held about another six months, but at that point the guard stopped us and said we could not discuss such matters."

John Dugard, secretary of the 900-member National Council of Lawyers for Human Rights, issued a statement saying: "Even if it be found that Dr. Neil Aggett took his life as claimed, the question remains--what makes death preferable to detention?"

After Biko's death, then-minister of justice Jimmy Kruger gave the impression in public statements that he had died of a hunger strike.

But the autopsy later showed Biko had suffered brain damage and other injuries.

At a judicial inquiry it emerged that the black consciousness leader had been kept naked and chained on a mat in his solitary confinement cell for several days in this injured state. He was then transported 650 miles through the night in the back of a jeep to Pretoria, where he died.

Police maintained at the inquiry that Biko was injured while attacking his interrogators.

But the lawyer for Biko's family, Sidney Kentridge, suggested strongly that the black consciousness leader had been brutally assaulted by his main interrogator.

Since the international outcry over Biko's death, the security police of the white-minority government have given repeated assurances that additional precautions are being taken to ensure that political detainees come to no harm.

Before Biko's death, 42 people, all of them nonwhites, had died in police custody. Several were said by the security police to have committed suicide or died in accidents.

Aggett is the first white political detainee to die.

On Wednesday, a government commission appointed to examine South Africa's security legislation recommended the retention of the laws allowing for indefinite detention without charges and the "banning" of people and organizations. Banning is the practice of limiting the activities of political opponents, sometimes placing them under house arrest or exiling them to remote regions of the country.

The commission said these practices were necessary because South Africa faced a threat of terrorism from organizations aided by communist countries and neighboring black-ruled African territories.

On Thursday the minister of police, Louis le Grange, gave an assurance in Parliament that "all reasonable precautions are taken" to prevent the 168 political detainees the government says are being held from "hurting themselves or being hurt in any other way or committing suicide."