Scores were killed overnight when fire tore through a derelict Johannesburg apartment block occupied by homeless people, officials and local media said on Thursday, as emergency evacuations from the blackened building continued.

The fire killed at least 73 people and injured 43, the municipal government said, in one of South Africa's worst such tragedies in living memory.

The blaze, whose cause was under investigation, started at around 1:30 am, Johannesburg Emergency Management spokesperson Robert Mulaudzi said.

At 10 am, the five-storey building was still smouldering, a large part of it blackened by soot, as emergency services gathered around it and bodies lay covered in blankets on a nearby street, a Reuters reporter said.

Municipal government speaker Colleen Makhubele implied those inside had been occupying it illegally, and that the city had sought but often failed to evict squatters from such buildings.

They had been to blame for "not heeding to the call of the city to do the right thing...," she told South Africa's public broadcaster.

"Things like this happen ... because the citizens are not willing to understand and work with us in upholding the laws and the bylaws of the city, in understanding what is safe, what is not safe."

Makhubele did not confirm a report by local broadcaster News24 that the building that caught fire was derelict and had been occupied informally.

Johannesburg is one of the world's most unequal cities with widespread poverty, joblessness and a housing crisis. It has about 15 000 homeless people, according to the provincial government of Gauteng, which includes South Africa's biggest city.

Fires are common there. It suffers from chronic power shortages during which many resort to candles for light and wood fires for heat.