Police said a bomb planted in a motorcycle exploded outside a hotel on Jinnah Road in Quetta.
Two paramilitary personnel and 10 passers-by were injured. Two of the injured are in a critical condition, hospital officials said.
Police and law enforcement personnel cordoned off the area immediately after the blast and launched a search for those responsible for the attack.
Officials of the bomb disposal squad said the blast was caused by an explosive device fitted with a timer.
The hotel's management alerted police about the suspicious motorcycle but the device went off before the bomb disposal squad reached the spot.
No group claimed responsibility for the blast.
Such attacks in southwestern Balochistan province are usually blamed on national groups waging a campaign for greater autonomy.
