Gunmen burst into a court in a busy shopping area in the center of the Pakistani capital on Monday killing at least 11 people, including a judge, police and witnesses said.
A loud explosion was heard in central Islamabad, followed by bursts of gunfire. Police said at least 30 were wounded.
"There was a blast, then there was a lot of gunfire. Gunmen were spraying bullets at everyone," said Faisal Ali, a businessman who witnessed the attack.
No one immediately claimed responsibility. Pakistani Taliban fighters stage frequent attacks around the country, but such assaults are rare in Islamabad, the leafy and hilly seat of Pakistan's government.
The attack also took place after the Taliban declared a one-month ceasefire over the weekend in order to pursue stalled peace negotiations with the government.
Rubble, broken glass and charred human remains littered the site of the blast at the F8 market area as residents and police rushed around in bloodstained clothes.
Police blocked entry and exit points to the area, a maze of narrow streets lined with shops and offices.
Pools of blood and severed body parts were scattered on the floor of several of the offices.