Charred vehicles and a mangled motorcycle lay outside Islamabad's district court Tuesday, their metal frames still warm as investigators sealed off the area where a suicide bomber had killed at least 12 people.

It was a rare assault at the heart of Pakistan's capital where blood stained a two-way street, and shattered glass glimmered among the debris.

"It was a very strong and terrifying explosion," said Khalid Mandokhel, a 24-year-old lawyer.

"Many of the victims were bystanders," he told AFP at the scene.

The blue police van hit in the blast stood crippled at the entrance, its metal pierced by shrapnel and tyres shredded, as investigators documented every mark on the vehicle.

More than 25 people were wounded in the attack.

Rustam Malik, another lawyer, said he "heard a loud bang at the gate" as he entered the complex, where daily, hundreds of people arrive to sort out legal matters. 

They did not expect to leave with their lives hanging by a thread. "There was thick smoke," Malik told AFP.

Interior Minister Mohsin Naqvi told journalists outside the court that the attacker had lingered outside the building for several minutes before striking.

"He could not go inside (the complex); he blew himself up near the police vehicle," Naqvi said.

'Complete chaos'

The complex, home to several courts, is usually one of the busiest areas in Islamabad during the day. Nearby offices of senior administration and police officials add to the dense crowds.

Outside the government-run hospital where most of the victims were taken, people were milling about as rescue workers wheeled people with head and leg injuries into the emergency room. Security guards, on alert, prevented the media from entering.

Back at the blast site, cordoned off at several points, investigators gathered evidence until late afternoon.

A heavy presence of police and paramilitary soldiers combed through the wreckage as a stray dog rummaged through the remains, sniffing at the scattered debris.

Malik recalled "complete chaos" --  lawyers and visitors running in panic after the explosion thundered through the compound. 

As investigators processed the scene, the lawyer waited patiently for police clearance so he could collect his vehicle which was damaged in the attack.

"I saw two dead bodies lying at the gate and several cars were on fire," he said.