Nervous South African security policemen testified today that black leader Steve Biko was transported naked and unconscious in a Land Rover for the 12-hour, 750-mile trip from his prison to a Pretoria hospital the day before he died.

The testimony, which came in the third day of an inquest into Biko's death, was hammered for discrepancies by the Biko family's lawyer, Sydney Kentridge.

Biko's death Sept. 12 while in police custody took a backseat for much of the time, however, as Kentridge raised serious questions about the practices and behavior of the security police in general, something rarely done in public in South Africa.

Kentridge directed his most acrimonious attack against Col. P. J. Goosen gazed with obvious exaspiration at the vaulted ceiling in the inquest room, sweat often pouring down him temples. About 500 spectators, mostly blacks and including Biko's family, cranded their necks to hear as Goosen answered in Afrikans.

Goosen told the hearing that although he noticed a swelling on Biko's lip, a "wild expression" in his eyes, incoherent speech and refusal of food, he thought Biko was taking in order to avoid answering questions. Kentridge has maintained that Biko suffered a head injury, probably during an alleged scuffle with interrogaters on Sept. 6.

"Can you explain why Biko had to sham illness to avoid answering questions?" Kentridge asked Goosen. "Why can't he just say, 'Good day gentlemen, I'm not answering any questions today?"

At another point, Kenridge asked Goosen why Biko had to be kept chained to an iron bar and handcuffed when "he had three policemen and iron grills all around him? Would you keep a dog chained up for 48 hours?" Kentridge went on.

"If I regarded him as absolutely dangerous I would have done so," Goosen replied.

Kentridge also spotlighted police practices as they came to the surface in discusing Biko's detention. Kentridge asked Goosen why Biko had been held naked in his cell. The police chief replied that "as a result of a clear pattern of suicide that has emerged over the past year," he had given instructions that all male prisoners be stripped so they do not use their garments to hang themselves.

Since March 1976 more than 20 men are known to have died in police detention in South Africa, according to church groups monitoring the jailings. South African security police have said that most of the deaths were suicides, committed mainly by hanging.

Goosen acknowledge that he told doctors who checked Biko that the prisoner was able" "to draw a veil" between himself and his interrogators and that Biko had studied eastern Yoga.

Kentridge inquired as to how Goosen knew these facts and Goosen replied, "It was generally known in security circles."

Kentridge said that Biko's family never saw him practice yoga, and he called the information "general gossip."

Goosen tried to evde an inquiry into why his men waited until Sept. 8 to record in the prison log the account of an alleged scuffle with Biko on Sept. 6 in which Biko may have hit his head. Presiding Magistrate M. J. Prins took up the line of questioning and asked, "Mustn't you automatically make a notation of these things?""No, it is left up to my discretion," Goosen replied. "It must be done at some time or another."

Goosen said he "did everything possible for Biko's welfare because I appreciate he was a person of considerabale importance. I was concerned to make every effort . . . to keep him healthy.Goosen added that he had done all this even though Biko "was nothing other than a terrorist leader, notwithstanding that the press projected him as nonviolent."

Perhaps the most damaging information against the police today was the suggestion by Kentridge that Goosen failed to alert the three doctors he called in to examine Biko to the fact that he had incurred a head injury. The doctors could find nothing wrong with Biko, and Kentridge suggested this was becuase they were never told of a head injury.

Even after Biko's death, when Goosen was aware that one of his top superiors was attempting to discover how Biko received his head injury, Goosen never mentioned in any of five sworn affadavits that Biko might have been injured in the alleged scuffle with his men. Instead, he suggested in the affadavits that he may have been injured during his temporary stay in the prison hospital.

One of Goosen's subordinates, Capt. D.P. Seibert, was one of the men who drove Biko to Pretoria. The security office in Pretoria had told Goosen to arrange to fly Biko in a military plane to the capital, but sinve it was Sunday and there were no planes, Biko was transported overland in the back of the land rover resting on a mattress and covered only by a blanket. He was given only supply water during the trip.

At the hospital, Seibert did not tell the medical oorderlies that Biko had received a bump on the head. Instead he gave background about Biko, including the allegation that he practiced yoga.

It was an attempt to again put the medical men off the scent . . . to persuade them that (Biko) was shamming his illness." kentridge charged, adding that Biko had been "smashed up" while police custody and "the one thing you (Seibert) never were prepared to say was that he had suffered a head injury."