The six-month inquiry into the death of union official Neil Aggett has turned into the most searching inquiry ever held into the interrogation methods of South Africa's security police.

The lawyer for Aggett's family, George Bizos, is taking an unorthodox approach to the case, trying to demonstrate that police drove the young white doctor and union official to suicide by brutal interrogation methods and prolonged solitary confinement.

Several former prisoners have taken the witness stand to testify about their own treatment under interrogation which Bizos says shows "a pattern of ill-treatment." Police interrogators who are used to operating without public scrutiny have had to submit themselves to interrogation by Bizos.

Among the most dramatic testimony was that by Auret Van Heerden, a former student leader who was held in the cell opposite Aggett's.

Van Heerden said Aggett whispered and made signs to him that he had been tortured and "broken" during 62 hours of nonstop interrogation, and that he "looked like a zombie" the day before he was found hanging in his cell Feb. 5.

Van Heerden said he recorded everything in a signed statement that he gave the officer in charge of interrogations, Maj. Arthur Cronwright. Cronwright told him he intended to suppress the statement and warned him not to say anything to the men appointed to investigate Aggett's death, Van Heerden testified. Van Heerden's evidence was denied by Cronwright and dismissed as a fabrication by police lawyer Pieher Schabort.

This type of testimony is unusual because South Africa's stringent security laws protect the security police from public scrutiny. They are not accountable to anyone except their own chiefs, the press cannot legally probe what they do, and, except when a prisoner dies, there is no recourse to the courts.

Even after a death -- and there have been more than 50 since the system of detention without trial was introduced in 1963 -- the inquests have usually been narrowly focused, with the presiding magistrates confining proceedings to the specific cause of death.

Police lawyer Schabort has strenuously resisted decisions by magistrare Petrus Kotze permitting testimony by former prisoners, reflecting the annoyance of the security police at the way the inquiry has turned an unaccustomed spotlight on them.

In June, Schabort appealed a key ruling by Kotze to the provincial supreme court in Pretoria, but lost. This cleared the way for the wider inquiry. It also highlighted the curious contradiction between South Africa's relatively independent legal system and the unbridled powers exercised by the security police.

The chief of the Johannesburg security police, Brig. Hendrik Muller, had to endure two hours of interrogation. When Bizos asked him why Aggett had been denied a visit by a magistrate, which the government says is one of the protections prisoners have, Muller snapped: "These people are detained for interrogation, not to be available for visits by magistrates."

The former prisoners Bizos has called to the witness stand were among 67 held for interrogation along with Aggett late last year during an attempt by the security police to establish whether there are links between the emerging black labor unions here and the outlawed African National Congress.

Eight former prisoners have testified that they were kept in solitary confinement for long periods before being taken to the 10th floor of the security police headquarters building in Johannesburg, where the interrogation rooms are.

They said their interrogators slapped, hit and humiliated them. They were made to stand for hours, sometimes days, and deprived of sleep. Some said they were stripped naked and made to do strenuous exercises, others that they were given electric shocks and had their genitals crushed.

Premananthan Naidoo, an East Indian, said he was allowed only a few hours' sleep during seven days of interrogation. He said he was given electric shocks, beaten, punched and kicked, made to stand for long periods and to kneel until the skin on his knee cracked, kept naked and threatened with death.

Naidoo said he became so exhausted toward the end that he fell asleep standing up and confessed to a crime he had not even been questioned about.

By the time Van Heerden took the witness stand, the magistrate ruled there had been enough evidence of other prisoners' treatment and he should confine himself to testifying about Aggett.

Despite this Van Heerden blurted out, before Kotze stopped him, that he had been made to stand hobbled, with his right wrist shackled to his left ankle, for two days and nights, and that he was strangled "to the point where I thought I was going to die."

Van Heerden said he and Aggett managed to communicate in whispers and signs through the grills of their cells when the guards left the outer doors open at mealtimes.

Early in January Aggett told him he had been made to strip and do exhausting exercises during an interrogation session. One interrogator had clubbed him about the body and face with his forearm wrapped in cloth.

Van Heerden said toward the end of January Aggett was taken to the 10th floor daily, culminating in his not returning to his cell from a Friday morning until Sunday.

All day Sunday Aggett's food was left untouched at his cell door and the next morning when he came to the grill he looked a changed man, Van Heerden said. He was downcast and tearful.

"He made a gesture with his hands as though snapping a twig and I heard him whisper, 'I've broken,' " Van Heerden said.

Aggett told him he had been forced to say he was a Communist, then he started crying.

During the following days Aggett's depression deepened, said Van Heerden, and by Feb. 4, the day before he died, "he seemed to have disintegrated and he looked like a zombie."

Van Heerden said he wrote out his statement after hearing that the attorney general of Transvaal Province had ordered an investigation into Aggett's death, and handed this to Cronwright.

"Cronwright told me he would not allow the statement to be submitted to the inquest. He said his men were doing a good job and he would not allow them to be persecuted."

Later, when the attorney general sent a lawyer to take statements from the prisoners, Van Heerden said Cronwright and three of the interrogators warned him that if he revealed what he knew about Aggett they would have him charged and jailed, and when he came out of prison he would be placed under house arrest.

He was warned that his conversation with the lawyer would be taped and the recording would be sent to Cronwright.

Van Heerden said he told the lawyer he had put all his observations in a signed statement. Back at security police headquarters Cronwright sent for him and said: "You were a fool to mention that statement because it does not exist."

Throughout the inquest the interrogators have denied mistreating Aggett or any of the other prisoners. They say their relationship with Aggett was good, that they joked, smoked and talked sport with him, and were astonished when he committed suicide.

He must, they contend, have been overcome with remorse after making a statement confessing to being a Communist and implicating friends.

Shabort suggested yesterday that Van Heerden had fabricated his testimony to reestablish his image among political radicals after rumours that he was a sellout and police informer.

Van Heerden denied this, saying: "If you say I am here to smear the police, all I can say is that the truth is often embarrassing.