The bomb was planted on a motorbike parked near the office of Balochistan National Party (BNP) in Sibi district. A meeting of the party was going on when the bomb exploded, they said.
The bomb was detonated through a remote control device and it damaged some parts of the office, they added.
No group has claimed responsibility for the attack. But Baloch nationalists often carry out such attacks. They want greater control over the resources of mineral rich province.