Eleven people were killed and 45 others were injured when a suspected suicide car bomber struck an office of a special investigation unit of police in Lahore on Monday.
The blast occurred at the building housing the investigation unit in Model Town area of Lahore at around 8.15 am.
Punjab police chief Tariq Salim Dogar described the blast as a "vehicle-borne explosion".
"I was informed that the terrorists came in a car and it exploded at the gate," he said.
Witnesses said an explosives-laden car had rammed into the building.
Javed Akram, a senior doctor at the Jinnah Hospital, said five persons, including a woman and a child, had died.
The hospital had received 45 injured people and nine of them were in a critical condition.
There were 13 women among the injured, he said.
"Some of the injured sustained multiple injuries. Doctors are performing emergency surgery on them," Akram told the media.
An emergency was declared in all hospitals in the city and doctors were recalled for duty.
The powerful blast flattened the two-storey building housing the special investigation unit and several people, including police personnel, were feared to be trapped in the rubble. It created a crater about 20 feet wide and eight feet deep.
The explosion also severely damaged several nearby houses and shattered windows of buildings located with a radius of 3 km.
"We were in the process of shifting police offices from residential areas but this attack occurred before we could complete the shifting of this unit," said Punjab police chief Dogar.
No group claimed responsibility for the attack.
Last year, terrorists had struck the office of the special investigation unit in another part of Model Town.
Following that attack, it was shifted to its current location. There are at least eight offices of various intelligence agencies in the area.
Sources said some important suspects with links to the Tehrik-e-Taliban Pakistan were being held in the office that was targeted on Monday.
However, there was no official word on these suspects and their status.
Terrorists have also targeted offices of the Federal Investigation Agency and Inter-Services Intelligence in Lahore over the past year, killing dozens of people.