What happens to South Africa's security prisoners?

The knock on the door usually comes early in the morning. Often, no one is notified of the arrest, and sometimes the press is precluded from reporting it once it is discovered.

Once behind bars, prisoners under South Africa's tough security laws lose virtually all legal rights. What happens to them? How are they treated?

Answers are being demanded in light of the death in police custody of a young black student, Moabi Dipale, found hanged in his cell Aug. 8.

As with the death of white detainee Neil Aggett six months ago, and the 45 preceding deaths of security prisoners in South African cells, this incident has raised suspicions over whether prisoners are being killed or induced to commit suicide by the police.

One leading anti-detention organization puts the number of deaths at 53.

Mr. Dipale was arrested last October, released without being charged, then arrested again three days before his death, according to a family lawyer.

The chief of security police, in announcing the death, said Dipale had hours earlier made a confession and was shortly to be brought to court for violations of internal security laws. The police are investigating the cause of death.

The ongoing inquest into the death of union organizer Neil Aggett has shed some light on how detainees are treated, and tends to confirm fears of those who find inadequate safeguards in South Africa's security laws.

Aggett made a statement to a visiting magistrate that he had been tortured by police. Another detainee corroborated the claim by saying he had seen Aggett being beaten during interrogation. Aggett underwent 60 hours of intensive questioning in the space of three days, and was denied visits on three occasions by officials meant to act as watchdogs over security police practices.

Critics of South Africa's security laws say abuses are possible because ''detainees have no rights under law.'' The system allows for the indefinite detention of people, without any right to a lawyer, at the sole discretion of the police.

''Nothing can safeguard lives of people held in those circumstances,'' says Helen Suzman, an opposition member of Parliament. ''They are basically incommunicado.''

South Africa earlier this year completed a major revision of its security laws. But critics saw no real progress in protecting the safety of detainees.

A recent examination of South African security legislation by the Center for Applied Legal Studies at the University of the Witwatersrand found considerable evidence of physical assault, psychological coercion, and degrading treatment of detainees at the hands of the police.

The report also said that while detainees are not held in special isolation cells, their incarceration resembles that of solitary confinement in that they are segregated from other prisoners, denied regular exercise, and prevented from the normal work routine afforded other convicted prisoners.

Suzman and the Progressive Federal Party have pressed the government to enact some code of conduct to regulate interrogation practices, but the government has refused to do so.

South Africa's ruling National Party justifies its extraordinary security measures as necessary to cope with the rising tide of internal violence as blacks increasingly challenge white minority rule.

But security experts point out that even terrorist-plagued states like Northern Ireland and Israel grant their police less sweeping powers than South Africa.

Those pressing for greater protection of detainees have urged as initial steps a judicial review of detentions and granting security prisoners the right to see their families, a doctor of their own choice, and a lawyer.