The blast at Bacha Khan Chowk in Quetta, the capital of the restive Balochistan province, was heard from several kilometres away.
The roundabout, located near several markets, was crowded when the bomb went off.
Officials at a nearby hospital said 11 people, including a child and a Frontier Corps personnel, had died.
They said they had received 27 injured, including several children and security personnel.
Police said the blast was caused by a timed explosive device that was planted under a car.
An estimated 20 kg of explosives was used in the attack, they said.
The blast caused a huge crater several feet deep. It destroyed about 10 cars, including a security forces vehicle, and several motorcycles.
The windows of nearby buildings were blown out and several shops were damaged.
Footage on television showed rubble and carts lying at the site of the blast as ambulances rushed the injured to hospital.
Some cars were hurled several feet by the powerful explosion. Security forces cordoned off the area and scoured the site for clues.
No group claimed responsibility for the attack.
Several militant groups, including the Taliban and Baloch nationalist organisations, are active in Balochistan.
Islamabad: A powerful bomb went off near a security check post at a crowded roundabout in the heart of Quetta city of southwest Pakistan on Friday, killing 11 people including a security personnel and injuring 27 others.
First Published: Thursday, January 10, 2013, 18:12