A Putco bus overturned on Sunday injuring 21 ANC supporters on their way to an election rally in Gauteng, Netcare 911 emergency services said.

Spokesman Nick Dollman said a red VW Jetta collided with the bus and ripped out its rear axle. The bus overturned and landed on its side at around 9.00am on Sunday.

One woman was in a critical condition after suffering severe head and facial injuries when she was thrown out of the bus during the accident.

The critically injured woman was airlifted to the Sunninghill hospital.

Twelve people had suffered serious injuries and eight others suffered minor injuries. They were taken to hospital by ambulance.

Five other people, including the driver, refused medical treatment for minor injuries and wanted to continue on to the African National Congress rally at Coca-Cola Park stadium in Johannesburg.

The driver of the red Jetta fled from the scene and a passenger in the car was taken to hospital by a private vehicle and not by ambulance.

Johannesburg metro police Superintendent Wayne Minnaar said a case of fleeing the scene of an accident would be investigated if the driver of the Jetta had run away.