The blast occurred in the parking area of the Judicial Complex.
“The suicide bomber targeted a police vehicle as he couldn't enter the complex,” Naqvi told reporters at the blast site. Efforts are underway to identify the attacker, he added.
President Asif Zardari condemned the attack, saying "foreign-sponsored" terrorists operating in Pakistan must be eliminated.
Security sources told Anadolu that the bodies of the 12 victims were taken to a local hospital, and others were transferred for emergency treatment.
Police and emergency teams cordoned off the area and launched an investigation.
Separately, security forces were engaged in an operation against three militants hiding inside a building at Wana Cadet College in northwestern Pakistan, security sources said.
About 500 cadets are housed in a hostel on the campus, located away from the building where the militants are holed up in South Waziristan, according to the sources.
Naqvi said three security personnel had been killed in the operation so far.