The attack which took place in the Chur Chowk area of the city was earlier referred to as a bomb blast, but later senior police officials confirmed that it was a suicide attack.
A suspected suicide bomber drove his motorcycle into a bus triggering a huge blast, RPO Rawalpindi, Nasir Durrani said.
The injured have been shifted to local hospitals and relief and rescue work is still on.
The blast also destroyed several vehicles parked near the bus.n emergency has been declared in the region.
The attack came hours after the Pakistani media had claimed that a top Taliban commander Mullah Fazlullah had been killed by security forces.