Tshwane mayor Smangaliso Mkhatshwa on Thursday bemoaned the virtual destruction of a monument to apartheid leader JG Strijdom in a building collapse in central Pretoria on Thursday morning.

I have learned with regret of the collapse of the Strijdom Square monument," he said in a statement.

"That monument was one of our most prominent landmarks, and a work of art. It is always a sad day when an irreplaceable artwork is lost."

A massive sculptured head of Strijdom tumbled into a hole in the early hours of Thursday as an underground parking garage collapsed.

The monument was virtually demolished. All that remained were sculptured horses on a plinth alongside the Strijdom head. The sculpture's head could not be seen from the side of the square, the centre of which had sunk several metres, as if into a sinkhole.

Fire brigade spokesperson Johan Pieterse said the head was lying at the centre of the hole. It appeared to be broken in two.

A huge retaining wall behind the monument buckled as a result of the collapse.

Two homeless people, a man and a woman, were injured in the collapse of the underground parking garage. The man was taken to Pretoria Academic Hospital and the woman to Kalafong Hospital.

Pieterse said both were able to talk to emergency workers before they were taken to hospital.

A Pretoria Academic hospital official said the man was in a stable condition, adding, "He was not so badly injured, and mainly suffered facial bruises."

A Kalafong hospital spokeswoman said, "The woman is fine. She did not sustain any serious injuries."

The collapse came on the 40th anniversary of what would have been "Republic Day" under the apartheid regime - marking the day South Africa left the Commonwealth to continue its pursuit of racial segregation policies.

Strijdom Square made headlines in November 1989 when rightwinger Barend Strydom, known as "Wit Wolf", killed seven black people and wounded 16 more in a 15-minute shooting spree on the square.

He was sentenced to death for the seven murders and another one he had committed a week earlier. Less than four years later, he was released after less than four years in jail as a result of negotiations between the then National Party government and the African National Congress.

A building housing offices of the Absabanking group on the southern side of the square, as well as the State Theatre on the eastern side, were closed to workers on Thursday morning.

"We first have the check that the structures are safe," Pieterse said.

Several shops around the square were also closed.

Police Sergeant Lolo Mangena said a Tembisa man, Vusi Mahlangu, who was at work at a nearby hotel, alerted police at about 4.40am.

"He told police he heard a bang and then saw the square sinking into the ground."

Emergency workers were by mid-morning still searching through the debris, and the square remained cordoned off. Streets adjacent to the area were closed to traffic.

Pieterse said it was unlikely that more people were trapped in the rubble. There did not appear to have been any cars parked in the garage at the time of the collapse.

The mayor said the cause of the collapse was not yet known.

"The police are investigating and we ask that there be no speculation until they have concluded their investigation," Mkhatshwa said. - Sapa