The blast occurred in a crowded bus at Pir Wadhai Chowk in Rawalpindi. Witnesses said at least nine persons were killed in the attack.

The injured were rushed to nearby hospitals by ambulances and the condition of four of them was said to be critical.

The head of the suspected suicide bomber had been found at the site, Aaj TV channel reported.

Many of the victims were hit by ball bearings that were packed into the suicide jacket and several vehicles were severely damaged by the blast.

Interior Ministry Chief Rehman Malik told reporters the suicide bomber could have caused widespread destruction if he had been able to target political workers and leaders who had gathered for a protest organised by lawyers and political parties.

Police officials said the bomber's explosives could have detonated prematurely before he could strike his intended target.

Security personnel cordoned off the area and began scouring the site of the blast for clues.

Malik said law enforcement agencies would remain on alert as intelligence reports had said there could be a "series of suicide attacks" on key buildings and important personalities.