Curfew was immediately imposed in northwestern Pakistan's Swat valley on Saturday morning as a blast occurred near a police station, killing at least 20 people, state television said.

The suicide bomber rammed a explosive-laden car into a police station in Charbagh Tehsil of Swat valley of North West Frontier Province (NWFP), PTV quoted the police as saying.

There are also paramilitary soldiers in the police station.

The injured were rushed to hospital nearby.

Reports said a spokesman for Tehreek-e-Taliban Pakistan (TTP), or Pakistan Taliban Movement, claimed responsibility shortly after the suicide bombing.

The TTP is believed to launch the attack to counter back the army's operation in the Swat valley.

This was the third blast since former Pakistani President Pervez Musharraf declared resignation on Monday.

The TTP also launched suicide attacks near two gates of the Pakistan Ordnance Factory in Wah Cantt in eastern Pakistan's Punjab province on Thursday, killing at least 70 and injuring dozens.

On Tuesday, a suicide bomber blew himself up at a hospital in Dera Ismail Khan of the NWFP, killing about 30.

(Xinhua News Agency August 23, 2008)