Apartheid-era police illegally held anti-apartheid activists Ahmed Timol and Salim Essop at John Vorster Square in order to torture them, the High Court in Pretoria heard on Thursday.

"They were held, in my view, illegally in the offices on the 10th floor. One can only come to the conclusion that it was for reasons of torture and it was for reasons of sleep deprivation," a private investigator commissioned by Timol's family, Frank Dutton, said.

Dutton was testifying at the second sitting of the inquest into Timol’s death, presided over by Judge Billy Mothle.

He said a detention order stated that Timol and Essop had to be kept in the cells on the third floor of John Vorster Square, but this didn't happen.

John Vorster Square is now called the Johannesburg Central police station. Earlier, he told the court that previous investigations were a cover-up.

"It was alleged by the police that a policeman identified only as 'Mr X' came into the room where Timol was being held and announced that he knew where Quentin and Henry Jacobsen were."

He said the officers allegedly said they had reason to believe that Timol and the Jacobsens were co-conspirators, which drove Timol to commit suicide by jumping out of the window.

Dutton questioned the existence of Mr X and said police never took a statement from him. He said security police wanted to remove Timol’s body from the scene as quickly as possible, to avoid public scrutiny.

Timol's death, on October 27, 1971, was ruled a suicide following an inquest in 1972. Police said he fell at 15:50.

His loved ones refused to believe the police’s version that he jumped and commissioned Dutton to investigate.

He subsequently uncovered new evidence which was presented to the National Prosecuting Authority, and the inquest was reopened.

Several witnesses have told the court they believed Timol was pushed.

Earlier, Don Foster, author of "Detention and Torture in South Africa", published in 1987, said detainees were physically and psychologically tortured.

He interviewed 158 people detained between 1974 and 1984, in major urban centres. "Detention is a highly stressful situation," he said.

Some told him how they were made to sit on imaginary chairs and confronted with false accusations.