Steve Biko had in death what he was denied in police custody as a political detainee - the services of what is regarded here as probably the best legal mind in South Africa today.

With unruly shocks of gray hair bursting from his temples, slight hands fidgeting behind his back and bushy eyebrows knitted in a permanent frown, white lawyer Sydney Kentridge, 55, has been the main actor at the inquest into the black leader's death during the past three weeks.

From proforma, sel-serving sworn affidavits and evasive security police witnesses. Kentridge eked out for public display, through withering cross-examination, details of mistreatment of Biko during the final days before he died of head injuries Sept. 12.

Kentridge's interrogations of police witnesses were the highlights of the often boring, repetitious inquest testimony, and the partisan crowd took delight in seeing the tables turned on the police.

His style, according to one of the police witnesses, is much like that of the security police. When asked how police do their interrogations, Col. Pieter Goosen told the packed hearing room that their style resembled that of Kentridge.

"First we talk nicely, then we get angry, then sarcastic . . ."

The slightly built advocate is not a newcomer to sparring with the security police. Although much of his practice involves lucrative civil litigation and patent cases, he has defended clients accused of political crimes on several occasions.

But, as one collegue said: "One goes to him in the most important political trials, not for the ordinary, run-of-the-mill political trial."

In the 1950s Kentridge was one of the defending lawyers for 156 people accused of treason. He also defended two journalists accused of falsely reporting about conditions in South African jails. And when the Angelican dean of Johannesburg was charged with being a terrorist, Kentridge was his lawyer.

Kentridge also took part in the investigations into the Sharpeville massacre of 1961, and his legal activities have brought him to the higher courts of France and Britian. In a British court he once argued, on behalf of two clients detained by the rebel Rhodesian government, that Smith's regime was illegal.

Kentridge chooses his political cases with the same care that he prepares for his cross-examination, and this has not gone unnoticed by the conservative Afrikaner community. One Africaner said that, although they "are inclined to think he's leftist himself and see him as a political advocate for those charged with leftwing (activity), they note that he doesn't take on any saboteur cases."

"To many of them," said one Afrikaner newsman, "he's got a devilish intelligence. Because he's so clever, they think it's not really justice that happens when he wins."

One of the young Afrikaner policemen on duty at the converted synagogue where the inquest was held said he didn't like Kentridge "because he's against us. He's out to get the police. If he can run you into the ground, he will. But then he tapped his closely cropped head and said: "He's got what matter up here though. He's a fantastic lawyer."

The blacks who milled about in the concrete courtyard of the old synagogue smiled broadly at Kentridge as he came and went into the inquest. "They idolize him," said one black reporter.

Another black, noting Kentridge's Jewish background, said: "We think that while he is on out side, he is also fighting his own battle with the Afrikaners by exposing the system."

Professional colleagues note that Kentridge can oversimplify when it suits him, but they attribute his success to an analytical brain, persuasive tongue and the fact that, as one friend said, "he's not intimidated by anyone - including judges."

Born in Johannesburg, Kentridge graduated from the University of Witwatersrand here, served in his country's armed forces during World War II and received his law degree from Oxford. He was a guest lecturer at Harvard University for six months. His father was a Labor Party member of South Africa's Parliament in the early days of the country's independence.

Kentridge himeslf, however, has no apparent political ambitions and outside of the courtroom he keeps a low profile. He declined to be interviewed, saying that professional ethics prohibited him.

But he is not averse to smiling at the television cameras, and he is not above the melodramatic. On the first day of the inquest he had the iron chains which Biko wore passed around the hearing room, clinking loudly.

Kentridge, who likes John Dean's "Blind Ambition" because "it's a very, very well-organized book," lives with his wife, Felicia, also a lawyer, and their four children in a huge white house in a wealthy suburb of Johannesburg.

The view from the home stretches for miles across the city and then to the rolling hills. It is an appropriate span for a man who sees further than most into the future. At a 1972 symposium on security legislation and justice, Kentridge shared the platform with the man who is now head of South Africa's Bureau of State Security and former head of the security police, Gen. H. J. van den Bergh. At that time, Kentridge told the student audience:

"I agree with Gen. van den Bergh that he and his men totally liquidated the Communist conspiracy in South Africa in the 1960s. The trouble is tha one of the inheritances we have since those days is a vast force of security police. I tend to feel some sort of modification of Parkinson's Law applies here. They have got to keep themselves busy . . . after you have got the real conspiracy, you look for others - perhaps people who are critics of the government or eccentrics of some sort."