Tehreek-e-Taliban Pakistan spokesman Shahidullah Shahid said the attack was in retaliation for the killing of their members by police and security forces in Peshawar, Swabi and Karachi, Geo News reported.
The spokesman said Taliban would carry out similar attacks until a cease-fire was announced.
Representatives of the Pakistani government and the Taliban have been in discussions about ending the explosion of violence plaguing Pakistan. Both sides have stressed a cease-fire was necessary for the talks to be successful.
The attack occurred near a police training center in the southern port city's Razzakabad area, Geo News said.
"Apparently it was a suicide attack as an explosives-laden car hit the police bus transporting officials," a senior police official said.
Xinhua News Agency quoted police as saying the bus was attacked as soon as it left the police training center. The policemen were on their way to take up duty at the residence of former Pakistani President Asif Ali Zardari.
Witnesses said the explosion was so powerful, the suicide attack vehicle was found more than 400 feet from the attack site.