A suspected underground gas explosion has ripped open roads and overturned more than 20 vehicles in South Africa.

Authorities said the blast during the evening rush hour in Johannesburg has injured at least nine people who were sent to hospital. Their injuries were said to be non-life threatening.

Meanwhile, others were being evacuated from the area amid fears of a second explosion or that multi-story buildings in a rundown part of the city could collapse.

Huge cracks and holes had opened in the middle of roads, some so big that vehicles slid down into them.

“Buildings are in danger of collapsing,” said Panyaza Lesufi, the premier of Gauteng province where Johannesburg is located. “The damage is extensive.”

Mr Lesufi said he had counted 23 vehicles flipped over by the explosion.

He said that gas was suspected to be the cause of the explosion but it was not clear if it was from a leak in the city’s underground pipes or from another, undetermined source.

Johannesburg gas supplier Egoli Gas said in a statement it was unlikely that one of its pipelines was responsible. The company said there was no interruption in its supply in the area and its investigators had found no leaks.

The explosion happened at 5pm local time as many people were gathering on the street to catch a minibus taxi home. Several minibus taxis and other cars were lying on their sides or backs, some even on top of each other.

Witnesses said people were already inside some of the minibuses when they were thrown into the air by the explosion.

One man, who did not give his name, told television station eNCA that he was in his car when he heard “a big sound”.

“The next thing, I was in the air and my car was overturning,” he said.

Emergency crews are still searching through some of the mangled, overturned vehicles and nearby buildings and Mr Lesufi said there could be more injured people. He said it was surprising but a relief that no deaths have been reported.

Mr Lesufi said rescue workers are worried about the amount of gas that had leaked out.

“This place is still dangerous,” Mr Lesufi said.

In the immediate moments after the explosion on the busy Bree Street in Johannesburg city centre, people were seen running as smoke poured out of a crack in the road.

Earlier this month, a toxic nitrate gas leak killed 17 people, including at least three children, in an informal settlement on the outskirts of Johannesburg. That was blamed on an illegal gold processing operation in the settlement.