The president of one of South Africa's main black trade unions, who is being held without charge by security police, has become the sixth political detainee in three months to be hospitalized.

Thozamile Gqweta of the Allied Workers Union was admitted to the psychiatric unit of a Johannesburg hospital last week, according to family members. His hospitalization came a week after the death in detention of Neil Aggett, a white trade union official.

At least five other detainees have been hospitalized, two for psychiatric treatment, since a police roundup of union officials in late November and early December, according to newspaper reports and accounts from family members. These mark the first reports of deaths or hospitalizations of detainees in almost four years.

Under South Africa's stringent antiterrorism laws, detainees may be held indefinitely without charge or trial and police are under no obligation to provide information about those being held. Police, who say Dr. Aggett hanged himself in his cell, have refused comment on the spate of hospitalizations. Concerning Gqweta, the police information department issued a statement saying only that he had been hospitalized "for observation" and that "his condition can be described as satisfactory."

Gqweta's elder brother, Robert Gqweta, said the family learned of his brother's hospitalization yesterday when they had made inquiries at police headquarters after hearing rumors.

Robert Gqweta said he was allowed to visit his brother yesterday and had been shocked by his appearance. Robert Gqweta said his brother, who has been held by police since Dec. 8, was suffering from a severe headache and loss of memory and had difficulty talking. He added that his brother seemed depressed and anxious and cried at one point.

"He was unrecognizable," Robert Gqweta said. "I had been allowed to see him in detention two weeks ago and the change was remarkable."

After visiting the hospital again today, Robert Gqweta reported that his brother's condition seemed improved. He said the visit had been conducted in the presence of two police guards, and that his brother said he had not been tortured.

There have been frequent allegations of torture since South Africa began 19 years ago to detain without trial people it suspects of political subversion. No case of torture has ever been proved, although the government has made several payments to settle damages claims before they have gone to court.

Fifty-two persons died in detention between 1963 and 1978, according to the U.S. State Department's latest annual report on human rights abuses, but police always have claimed death was due either to suicide or accident.

After an international outcry in 1977 when black leader Steve Biko died of brain injuries while in detention, there appeared to be an easing of conditions.

But conditions appear to have worsened following a new wave of detentions, which began in November. What is involved appears to be an attempt to link some of the main black labor unions here with the outlawed African National Congress.

Hospitalizations were first disclosed in late November, when newspapers discovered two women detainees had been admitted under the names "Mrs. Black" and "Mrs. Brown."

"Mrs. Brown" turned out to be Khosi Mbatha, wife of a field worker for the Catholic Bishops' Conference. She had suffered a heart attack in detention. "Mrs. Black" still has not been identified.

Following Aggett's death earlier this month, his girlfriend, Elizabeth Floyd, a union activist who had been detained at the same time he was, was admitted to a psychiatric unit. So was a detainee identified as Esther Levitan.

Another detainee, Rene Roux, also was hospitalized last week for unknown causes.

Gqweta and his union long have been the object of security police attention. The union operates in Eastern Cape Province, which was Biko's home base, and the blacks there are highly politicized.

Gqweta, a young, articulate union spokesman who bears a strong physical resemblance to the late black leader, is often referred to as "the second Biko."