The explosion took place in Karachi's old Sabzi Mandi area targeting a Station House Officer, said Shahid Hayat, the Additional IG Sindh.
Two people were killed and four police personnel are among the 25 people injured in the attack, he said.
Police suspect the involvement of the Tehreek-e-Taliban in the attack as several key TTP activists have been arrested from the area, a stronghold of the militants.
Hayat said the blast was carried out by parking a motorcycle laden with nearly five kilogram of explosives.
The explosion was so powerful that it shattered windows of nearby shops and houses and also injured many of the passerbys, vendors selling fruits and pedestrians.