South African police are searching for a man who shot dead three people on a bus in the capital, Pretoria, in a suspected racist attack on Wednesday afternoon.

A white man randomly fired bullets at black commuters on a bus bound for a township on the outskirts of the city.

Police used helicopters as part of a large-scale manhunt, but no arrests had been made by Thursday morning.

Four people were injured in the shooting, which took place as the bus was on its way through a mainly white residential area.

Two of the victims remained in hospital on Thursday, one of them in a critical condition.

Witnesses said the gunman appeared to have targetted the number 55 bus route, which carries commuters to the black township of Mamelodi.

They said he had allowed other buses to pass before boarding the Mamelodi bus.

'Point blank range'

He first shot the driver, then opened fire on other passengers, witnesses said.

"The gunman stared into my eyes and then shot the two men in front of me...bam bam...just like that," passenger Matthews Chauke told the Pretoria News.

"There was nothing in his eyes, just blank.

"He then turned to his right and shot the woman sitting there.

"They looked at him for a brief second and as their mouths started to open to scream he shot them at point-blank range.

"I dived under the seat and I saw no more."

Call for calm

South African newspapers compared the attack to the 1988 shooting by white supremacist Barend Strydom, who killed eight black people in central Pretoria.

President Thabo Mbeki appealed for calm, and urged Pretoria residents not to take the law into their own hands.

"The colour of this person and the victims, for our immediate purposes, is not important," said presidential spokesman Parks Mankahlana.

"This is a South African that killed and injured other South Africans.