Meanwhile in Baghdad at least eight people died in a truck bomb attack on a key bridge in the south of the city. The driver of the pick-up detonated his bomb near a checkpoint on the Jadariya bridge over the River Tigris. The bridge appears to have been left intact.

This follows a truck bomb which partly destroyed the Sarafiya bridge in northern Baghdad on Thursday, killing eight people.

The attacks come as a US-led security operation enters its third month. The surge has brought down the rate of sectarian murders but has failed to stop bomb attacks.

In Karbala, witnesses said police fired into the air to disperse crowds and clear the roads near the bus station, which had been particularly busy at the time of the attack.

Iraqi television aired footage in which rescue workers could be seen evacuating casualties. As many as 15 ambulances were said to be at the scene. Hundreds of people were seen gathered around the ambulances pounding their chests in grief.

Demonstrations followed outside the Karbala governor's offices. Protesters demanded the resignation of the governor and provincial council members, blaming them for security lapses. Police imposed a curfew and sealed off entrances to the city.