The explosion occurred at Sumangli road in Quetta, the capital of Balochistan province.
Officials at the site told media that four persons were killed and several others, including security personnel and passersby, were injured.
The security forces' vehicle and two other cars were destroyed by the blast.
Footage on television showed the cars burning as security forces cordoned off the area.
The explosion also damaged several nearby buildings.
No group claimed responsibility for the attack though such incidents are usually blamed on Baloch insurgent groups.
