Pakistan has been in the grip of a bloody homegrown Taliban insurgency since 2007 but attacks within the capital have been very rare in recent years.
"There was firing by two to three people followed by two suicide blasts which killed 11 people and wounded 24 others," Islamabad police chief Sikandar Hayat told reporters.
"All the attackers fled though one sustained injuries in the leg and back."
An AFP reporter at the scene saw blood and human remains in the court complex.
The death toll was confirmed by other police officials and the spokeswoman for the Pakistan Institute of Medical Sciences (PIMS) Ayesha Isani.
Isani said 20 wounded had been brought to PIMS, half in a critical condition.
Roads around the court, located in a well-heeled residential sector of the city popular with foreign residents, were sealed off as police and paramilitary forces carried out a search operation.