There is mounting evidence that Steve Biko, founder of South Africa's black-consciousness movement, suffered multiple brain and body injuries before his death in police detention Monday, according to well-informed sources here.

Although pathologists representing both the governemnt and the Biko family have said they will not complete the final post-mortem tests for several days, there is already enough evidence to indicate that Biko, 30, probably died of brain damages, such as a hemorrhage, medical experts and well-placed sources said today. There is also some suspicion that because of his injuries, Biko might not have been fully cognizant or conscious during the period before his death.

This information was made available within hours after Minister of Justice Jimmy Kruger's "clarification" on the widely reported eight-day hunger strike as the cause of death.

"I never suggested at any stage that Mr. Biko starved himself to death. I gave categorically the fact that he had gone on a hunger strike.That was given to me by police. I never said that was the cause of death," Kruger said in an interview with the South African Press Association. People skeptical of the government's version had contended that a man Biko's age would not die after a short fast.

Lawyers involved in the Biko case refused to comment on the implications of the preliminary findings, or how the injuries were sustained. But qualified observers queried today suggested that the government's position could become "far more uncomfortable, difficult."

"The findings make the case much more complex because of the possible causes of that condition - assult, negligence, self-inflication or illness," one source commented. "Determining the cause will be the focal point of this case".

Yesterday, Prime Minister John Vorster indicated that there was no need for a judicial inquiry into Biko's death because the normal process - an inquest - provides for a complete and fair investigation.

Several South African politicans, newspapers, academics and church, student and black groups have called for a judicial inquiry - conduct on a broader scale by a magistrate rather than a civil servant - to probe the facts behind the death of one of the country's most widely respected black leaders.

Last night, however, the minister of justice referred for the first time to the possibility of criminal proceedings. He did not elaborate or explain what led him to consider this possibility.

Biko was the 21st detainee to die in police custody in South Africa during the past 18 months.His death has brought to a peak the growing skepticism among both blacks and whites about the alleged causes of all detainee deaths, which the government has attributed mainly to heart attacks, suicides or escape attempts.

Biko, honorary president of the Black People's Convention and president of the South African Students' Organization, was detained Aug. 18 under Section 6 of the Terrorism Act near his home in King Williams Town. Section 6 allows detention without trial for up to one year.

Biko had not been charged with a specific offense, although Kruger disclosed yesterday that the government had planned to charge him with drafting and distributing pamphlets inciting people to arson and riot.

Biko became "unwell" at an unspecified time and was transfered from King Williams Town to a Pretoria prison Sept. 11. He died the following day, according to the government. Kruger said Biko went on a hunger strike Sept 5.

More than one physician suggested, however, that the apparent hunger strike might not be inconsistent with brain damage. The injuries could have resulted in a medical state known as trauma with symptoms of stupor, confusion and apathy which could have led to disinterest in or unawareness of food and drink.

"Many cases of trauma are not diagnosed correctly or completely under the best of circumstances. It could be the extent of his condition was not recognized and his guards thought he was deliberately refusing food." one Johannesburg physician explained."But that still leaves in question the key issue - the cause".

The preliminary postmortem results, once released, are certain to fan the mounting anger of moderates, both black and white, who believe that foul play by security police may have been a strong factor in the Biko case.

Meanwhile, rallies and memorial services to pay tribute to Biko are scheduled throughout South Africa Sunday. The Weekend World Johannesburgs African paper, said today that the "services are expected to be the most extensive for any black South African since the death of African since the death of African National Congress leader and Nobel Peace Prize-winner Albert Luthuli - More than 10 years ago."

One of the major church services will be held in Soweto, Johannesburg's troubled African township. Police have been put on alert in case of a new outbreak of violence.

Even whites have become actively involved in the protest. A young white member of Parliament, Graham McIntosh, has pledged that he and his wife will launch an eight-day "fast of protest".

"I am a man the same age as Steve Biko and I want to prove that I will not be dead after eight days without food."