Sharjeel Memon, information minister of Sindh province, said that the blast took place on Burns Road in central Karachi while Judge Maqbool Baqar Hussain and his convoy was on way to the Sindh High Court.
The bomb, which contained an estimated six to eight kg of explosives, was planted on a motorbike parked by the roadside and was triggered off through a remote control device while Baqar's convoy was passing by.
Six policemen and a Pakistan Rangers man were killed in the blast and 15 others, including seven to eight security personnel, injured.
The injured people were shifted to the civil hospital in Karachi where at least seven of them were in critical condition, the medical superintendent of the hospital said.
Deputy Inspector general of Police Ameer Sheikh said Judge Baqar has been shifted to Agha Khan Hospital and his condition is critical.
No group has claimed responsibility for the attack yet.
Both President Asif Ali Zardari and Prime Minister Nawaz Sharif condemned the blast and directed the hospital administration to give the injured the best medical treatment.