A gas tanker stuck under a bridge has exploded in Johannesburg, killing eight people, injuring scores more and damaging nearby buildings, according to emergency services.

The blast, on Saturday, damaged the roof of the emergency department at the Tambo Memorial hospital.

It destroyed two houses and several cars, and injured bystanders.

The tanker caught fire while under a low bridge, emergency services spokesperson William Ntladi said.

"Apparently, a gas tanker drove under the subway bridge and got stuck in there, and due to friction, it caught alight," Mr Ntladi told South African media.

He said six firefighters from Boksburg Central Fire Station were called to the bridge to help.

"While doing the fire suppression, there was an explosion that took place," Mr Ntladi said.

He added that a fire engine and two motor vehicles were destroyed by the blast.

The injured were transported to the nearby Tambo Memorial hospital, but its efforts to treat the injured were hampered by damage to its roof.

A video on Twitter from inside the hospital showed a chaotic scene, with people running for cover.

"We can confirm that a roof section at the hospital casualty department has collapsed," Mt Ntladi said.

Fire crews were still fighting the blaze at the bridge and were also attending to two homes nearby, Mr Ntladi said.

"The fire is now under control," he said.

Wires/ABC