Two militants threw hand grenades, fired guns and then blew themselves up at the district court building, said Sikander Hayat, inspector general of Islamabad police.
Witnesses at the court building told CNN they saw two men firing indiscriminately at people. Police officers, including an anti-terrorism team, are now at the scene.
Attacks of this nature are rare in Pakistan's heavily policed capital.
The Pakistan Institute of Medical Sciences, a complex that includes a hospital, reported the death toll of 11 and said 39 people were wounded in the attack. The dead included a judge, authorities said.
There was no immediate claim of responsibility for the attack on the building, which is in an upmarket residential district of central Islamabad. The area around the court includes supermarkets, restaurants and coffee shops.
The Pakistani Taliban, which on Saturday declared a month-long ceasefire, denied any involvement with the attack.
Shahidullah Shahid, a spokesman for the extremist group, said that the ceasefire was still in effect and that action would be taken against any faction that tried to break it.
