Two police officers were killed and two others injured when a bomb ripped through a police van while it was on patrol in north-western Pakistan.
The attack took place on the outskirts of the city of Peshawar, which is located on the edge of Pakistan's tribal region, the main sanctuary for Taliban militants in the country. The city has been hit by frequent bombings over the past few years.
It was unclear if the blast was caused by a roadside bomb or a suicide bomber, said senior police officer Javed Khan. The explosion completely destroyed the police van, he added.
Meanwhile, a gunman riding on a motorcycle killed the leader of an Islamic seminary in the southern city of Karachi, sparking a riot by hundreds of seminary students who stoned shops and set vehicles on fire, said police officer Azhar Iqbal.
Karachi has a long history of political, sectarian and ethnic strife, which has been on the rise this year.