A fire that tore through a five-storey building taken over for illegal housing killed 74 people including 12 children in central Johannesburg overnight, South African authorities said on Thursday.

An additional 61 were injured and treated in hospital in one of the deadliest fires worldwide in recent years.

Bodies were discovered piled up at a security gate that was closed, preventing people from escaping the blaze, an official said.

Thembalethu Mpahlaza, the head of forensics services in Johannesburg's Gauteng province said a total of 74 bodies were recovered, 24 female, 40 male and 10 "burnt beyond recognition".

"We are having 12 children involved also in this tragedy," he told a press conference.

City authorities said the municipality-owned building in a deprived, crime-ridden area had been turned into illegal housing after being abandoned.

Most of those living there were foreigners, one resident said.

"I'm grateful to be alive, there was a lot of us running, trying to find the fire exit and a lot of people eventually died because of the smoke inhalation," said Kenny Bupe, a survivor caught up in the blaze while visiting a friend.

The 28-year-old said he was part of a group that managed to break open a locked fire escape gate and run to safety, while others "jumped out" of windows to escape the flames.

Blankets and sheets used for escape hung from the burned-out windows.

Witnesses spoke of parents throwing their babies out into the street, hoping to save them.

"There were people catching the babies and there were also mattresses laid out for (them)," said Mac Katlego, 25, who lives across the street.

Rescuers combed the building floor by floor as firefighters damped down hotspots after putting out the flames.

"This is a great tragedy, felt by families whose loved ones perished in this terrible manner," South African President Cyril Ramaphosa said at an event in the southern city of Gqeberha.

"Our hearts go out to every person who is affected by this disaster."

Emergency services laid dead bodies under blankets on the street outside.

It was not immediately clear what caused the blaze. House fires are fairly common in the country, plagued by chronic power outages, where many live in poverty.

Authorities said candles used for lighting inside the structure or stoves and other heating devices were a likely cause. (AFP)