JOHANNESBURG, South Africa (AP) _ Police on Thursday fired tear gas at protesting students who blamed South Africa for the plane crash that killed Mozambique's President Samora Machel.

The Prisons Service, meanwhile, said a 20-year-old man was found hanging in his cell, becoming the 60th person since 1963 to die while detained without charge for security reasons.

Machel was killed Sunday night when the Soviet-made plane carrying him from Zambia to Mozambique crashed just inside South Africa. Neither South Africa nor Mozambique have commented on the cause of the crash, which killed 34 people.

South African newspapers have said poor weather and pilot error might have been to blame. The official Mozambican News Agency, however, quoted a Machel bodyguard, one of 10 survivors, as saying he heard the plane screech and ''it seemed that it was shot at.''

About 1,000 students, half of them blacks, attended a memorial service at the University of Witwatersrand for Machel. Speakers at the service repeatedly accused South Africa of being responsible for the crash.

After the service, about 100 students marched through campus and headed for the streets of Johannesburg.

Deputy Vice Chancellor Mervyn Shear pleaded with the students not to leave the campus, and they stopped short of the boundary. The youths ran as about 30 police entered and gave the order to disperse under a law barring outdoor gatherings.

The protesters regrouped and after a standoff, police fired three tear gas canisters at the students who were chanting slogans and singing freedom songs.

There were no reports of arrests or injuries.

At the University of Cape Town, about 350 students attended a memorial service for Machel.

In Johannesburg, about 400 people gathered at the offices of the Azanian Peoples Organization, an group that rejects white help in fighting apartheid.

Speakers blamed South Africa for Machel's death.

Similar demonstrations also have been held in Lesotho and Zimbabwe.

Soviet and Mozambican aviation experts were expected to fly to the crash site Friday, said Foreign Ministry spokesman Awie Marais.

Japie Smit, director of civil aviation, said the flight data recorder and the cockpit voice recorder have been recovered from the plane but have not yet been inspected.

In other developments, the Prisons Service announced the death of Xoluso Johannes Jacobs, saying he was found hanging by his shirt from bars in his prison cell Wednesday night. Jacbos had been held in Upington in northwestern South Africa.

Jacobs' race was not disclosed, but his name suggested he was black or of mixed-race.

Max Coleman, a leader of the Detainees Parents Support Committee, said Jacobs was the 60th person since 1963 to die while being held under emergency powers.

Coleman has estimated that about 20,000 people have been detained since the government imposed a national state of emergency June 12 to quell anti- apartheid protests.

Apartheid establishes a racially segregated society in which South Africa's 24 million blacks have no vote in national affairs. The 5 million whites control the economy and maintain separate districts, schools and health services.

In other developments:

-The Prisons Service said a black death row prisoner in Pretoria hanged himself Wednesday night, using his clothing.

-Elize Botha, the wife of President P.W. Botha, visited Vladimir Novoselov, a Soviet crew member injured in the Machel plane crash, at a Pretoria hospital. -A Johannesburg court sentenced Anton Werner Stoop, a 23-year-old white man, to death for the murder of a black mine worker. The victim was repeatedly beaten and then set on fire to make it look as if he had been ''necklaced,'' or burned to death by fellow blacks, according to trial testimony.