A bomb ripped through a police investigation building in Pakistan's biggest city of Karachi late Thursday, wounding more than 25 people, a senior police official said.

"More than 25 people have been wounded. Most of the injured are policemen," senior police official Tariq Razzaq Dharejo told AFP.

"It was a bomb blast," he said.

Local TV channel GEO broadcast footage of the site showing rescue workers ferrying people on stretchers into ambulances, dazed civilians stumbling in the street and a mass of twisted metal.

"The target was the office of CID (Crime Investigation Department)," Fayaz Lughari, senior police official, told AFP.

The two-storey building collapsed and several people are trapped under the rubble, an AFP reporter said.