Car bomb kills 12 Pakistan officers and wounds 55 people
As the bus drove by a parked van, the van exploded, said police. Initial investigation suggested there was a remotely-detonated bomb on the van.
Nearly 50 officers on board the bus were from the Rapid Response Force, a special anti-terrorist police squad. Most of the wounded also were officers on the bus.
Pakistani TV channels showed the blast scene littered with broken glass and pieces of vehicles scattered around. The wounded were rushed to nearby hospitals, and 12 of them were in critical condition, said doctors.
The attack took place in the port city of Karachi, which has long been a centre of ethnic, political, militant and sectarian violence.
Shahid, the Taliban spokesman, said the militants carried out the bombing to avenge the killing of their associates at the hands of police and paramilitary rangers. He said that until there was a formal ceasefire in place with the government, the Taliban would continue such attacks.
The assault is the latest in a series of attacks at a time when the Pakistani government is trying to strike a peace deal with local Taliban militants fighting in the country's north west to end the violence that has killed thousands of security forces personnel, government officials, political workers and civilians in recent years.
The Pakistani Taliban have been waging a bloody war against the government in a bid to overthrow the authorities and enforce their harsh brand of Islamic Shariah law.