The final autopsy report on South Africa's best known black power leader, Steve Biko, concluded that the cause of his death Sept. 12 while in detention was "extensive brain injury," it was learned today.

The just completed official state autopsy found that as a result of brain damage, the circulation of blood through Biko's body was seriously affected, leading to blood clots and ultimately to acute kidney failure, informed medical sources said.

Speculation that Biko was tortured by electric shock is apparently unfounded. There was reportedly no evidence of such torture, although there were indications of blows to the left side of the chest in addition to the head injury.

The findings seem to lend considerable weight to still unproven allegations that Biko died as a result of a beating at the hands of the South African security police. If so, the political consequences for the South African government both at home and abroad are likely to be considerable.

Justice Minister Jimmy Kruger, the man at the center of the international controversy over Biko's death, has suggested that there was a struggle at some point between the 30-year-old black activist and the police during an attempt to handcuff him. This may be the line of defense the state will take in attempting to explain the cause of his death.

The report is certain to increase the already enormous pressure on the government to hold an official inquiry into Biko's death.

The extensive autopsy report was written jointly by the chief state pathologist, Dr. Johann Goubser, and one appointed by the Biko family, Kruger said Monday night that both sides had concurred unanimously in the findings which he sent to Transvaal Attorney General J.E. Nathling to decide what action should be taken.

"If he should decide a judicial post mortem inquiry is necessary, I shall arrange for a court to be made available as soon as possible," Kruger said.

Two avenues of action are understood to be open to the attorney general: an inquest or criminal procedure probably involving charges of culpable homicide against those allegedly responsible for Biko's death.

The Biko family lawyers are said to prefer an inquest since this would permit them to freely call and cross-examine state witnesses, whereas criminal action by the state against its own security officials would preclude this.

In the past, the government has charged other security officers with culpable homicide and then settled the cases without the full truth ever being disclosed in court.

Biko's wife said in an interview today that she was counting on an inquest so that the full truth would come out.

"I do believe there will be truth out of an inquest," Ntsiki Biko said, expressing deep doubt about Kruger's suggestion that there was a scuffle in an attempt to handcuff her husband. "He was not that kind of man . . . He was kind of person who liked to sit down and discuss (problems) freely with people," she said.

"I believe they must have beaten him themselves," she said adding that she was planning to sue the justice minister for a figure in the hundreds of thousands of dollars.

She reported that one son, Nkosinethi 6, was still "morbid" about his father's death and the other, Samora, a 2-year-old named after Mozambique President Samora Machel, was "still lost" and was not really aware of that had happened.

The autopsy reportedly concludes that Biko died of a head injury of a "contra coup type," meaning one caused by a blow on the exact opposite of the head from the point of worst damage.

The injury is said to have resulted in the reduction of blood circulation to various organs of the body and ultimately caused the failure of Biko's kidneys.

This helps to explain why Biko was said by Kruger to have acted stranely, rejecting his food and water as if on a hunger strike during the last days of his life. The brain injury apparently took a number of days before it began affecting other parts of his body.

One conclusion that both the state and the Biko family lawyers are likely to agree upon is that the black leader was not given adequate medical attention prior to his death. Kruger has already virtually conceded such neglect by prison authorities.

Various doctors were reported to have examined him in the days immediately preceding his death without being able to determine precisely what was wrong with him or how to treat his symptoms.

Whethr he might have been saved had a correct diagnosis been made will probably never be known. It now remains to be seen whether the truth surrounding his death can be determined through either an inquest or a criminal proceeding against those responsible for his safety and interrogation while in detention.

News agencies reported these other developments involving souther Africa :

Zbigniew Brzezinski, President Carter's national security adviser, warned that the racial conflict in southern Africa could escalate into an ideological war with external powers involved.

In a speech delivered in Bonn, West Germany, Brzezinski said the United States supported the principle of majority rule and one-man, one vote in the region.

The United States recognized that the situation in South Africa was complex and would take time to resolve, Brezezinski said, but these must be "a new reality - one more in keeping with the spirit of the times," he said.

In Bloemfontein, South Africa, the trial of black militant Winnie Mandela, accused of breaking the terms of her banishment last May to a remote South African township, reopened.

Mandela, 43, wife of imprisoned nationalist leader, Nelson Mandela, has pleaded not guilty to the charges, which include receiving visitors and attending gatherings. If found guilty, she could be sentenced to up to 10 years in prison.