At least 22 people were killed and scores injured on Thursday in a suspected suicide attack on a Shiite Muslim procession marking Muharram in northwestern Pakistan. This set off riots with angry mobs torching shops and vehicles, prompting authorities to impose curfew.

A series of explosions ripped through the procession in the main bazaar of North West Frontier Provonce's Hangu town, some 200 kilometres northwest of Islamabad, between 9.30 am and 9.40 am.

The situation became tense as angry mob burnt shops and vehicles and also fired in the air. Interior Minister Aftab Ahmed Khan Sherpao said the army had taken control of the area and curfew was imposed to check further violence.

Paramilitary forces had also been deployed and they had taken position in the nearby mountains, overlooking the town, the minister told Geo TV.

The Minister said that angry mob burnt 20 shops and fire fighters were sent there to extinguish the blaze. He added that the main bazaar was cleared of people in a few hours and the army was in full control of the situation.

However, the minister confirmed only 12 deaths in the attack.

About the nature of the blast, he said there were two possibilities -- that a timer might have been used or it was a suicide attack. "Investigation is underway and the nature of the attack will be clear after the probe is completed," he said.

No one has claimed responsibility for the attack so far