A 20-year-old black man plunged to his death from the fifth floor of a police station Monday, becoming the 23rd person to die since March 1976 while in the custody of South African security police.

A statement released yesterday by Justice Minister Jimmy Kruger said Lungile Tabalaza jumped from a fifth-floor window of the Sanlam building in Port Elizabeth.

The building houses the same security police offices where black consciousness leader Steve Biko received the head injuries that resulted in his death last September.

The police announcement yesterday that Tabalaza, 20, had committed suicide was greeted by blacks with open disbelief.

Opposition members of Parliament called for an urgent investigation of Tabalaza's death. Opposition spokeswoman Helen Suzman termed it "particularly significant" that the death had occurred at the place Biko had been detained.

"This alone calls for special investigation into the methods used by the men in charge," she said.

Kruger said an inquest would be held as soon as possible at which relatives and their representatives could be present.

Kruger also said he had appointed a senior police officer to investigate Tabalaza's death "with a view to possibly disciplinary action . . . in view of the strict instructions that police should do their utmost to prevent detainees from committing suicide."

In his statement, Kruger said Tabalaza was arrested Monday by the uniformed police in connection with three incidents involving arson and robbery that he was not being held under South Africa's security laws, which permit indefinite detention without trial, and that he would have been formally charged within 48 hours.

Nevertheless, Tabalaza's death occurred after he had been handed over to the security police for further investigation, Kruger related. He said Tabalaza jumped to his death about 3 p.m.

An eyewitness, Marthinus Pretorius, who was working in his offices underneath the police headquarters, said he heard a thud outside his window and when he went out to investigate, found the black detainee lying in a pool of blood on the sidewalk. Pretorius said he sent for a blanket, which he gave to police who came to stand around the body.

"I was ordered [by the police] to go back to my shop," Pretorius said. Tabalaza died in an ambulance on the way to the hospital, Kruger said.

The justice minister's response to Tabalaza's death contrasts sharply with his attitude after Biko's death, when he initially reported that the black leader died of a hunger strike. Kruger later came under severe criticism domestically as well as internationally for his handling of Biko's death and for remarks belittling the event.

Last May, Kruger issued new rules to security policemen instructing that interrogations should be conducted in strengthened rooms unless they were on the ground floor. The rules explicitly stated that "everything possible had to be done to prevent detainees from jumping out of buildings."

Tabalaza is the first detainee to die since the inquest into the death of Biko last November.

The verdict in that inquest exonerated the police, who said Biko's head injuries were suffered during a scuffle. Biko's lawyer charged that the dead black leader had been beaten by police, and then denied proper medical treatment.

The Biko verdict drew criticism at the time on grounds it would be interpreted by the police as a license to use whatever means they wanted in interrogating prisoners. Police deny they use physical means to elicit information.