The nine were sent to the hospital with injuries that authorities said were not life-threatening. Other people were evacuated from the area due to fears of a second explosion or that multi-story buildings in a downtown section of the city that has become run down in places might collapse.

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

Authorities estimated that an area covering five city blocks was damaged. Lesufi said he counted 23 overturned vehicles. Huge cracks and holes appeared in the middle of roads, some so big that vehicles slid down into them.

Lesufi said that while gas was suspected as the cause of the explosion on downtown Johannesburg’s busy Bree Street, it was not clear if it came 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 5 p.m. explosion happened as many people were gathering on the street to catch a minibus taxi home, one of South Africa’s most common commuting methods in cities. Several minibus taxis and other cars were left on their sides or flipped over, with some lying on top of another vehicle.

Eyewitnesses said people were already inside some of the minibuses when the explosion threw them into the air. In the immediate moments after the blast, people were seen running as smoke poured out of a crack in the road.

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. He said he was shaken but unhurt.

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

Lesufi said rescue workers were worried about the amount of gas that had leaked out as a distinct smell of gas hung in the area.

“This place is still dangerous,” he said.

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. The leak was blamed on an illegal gold processing operation in the settlement.