Mian Bashir Bhatti, a judge of the anti-terrorist court who often hears cases against militants and suspected terrorists, was traveling to his court in central Multan when the bomb went off, said Shaukat Ali, the city police chief.
In addition to the judge, seven other people were wounded in the blast, one of whom later died in a hospital, Ali said, without identifying the man.
Munir Chishti, the district police chief, said the bomb was tied to a bicycle parked on a road, and that the device was remotely detonated by "terrorists" to target the judge and the police.
He declined to say who might have been behind the attack.
Dr. Fahim Javed at the state Nishtar Hospital said Bhatti was in a critical condition, and identified the other two who died as Mohammed Iqbal, Bhatti's driver, and Ijaz Ahmed, a policeman who was escorting the judge's car on a motorcycle. Both died at the scene of the attack.
Mirza Mohammed Ali, another police officer, said Bhatti was hearing the case of Malik Ishaq, a leader of the outlawed Sunni Muslim militant group, Sipah-e-Sihaba, which is implicated in sectarian attacks against Shiites.
Ishaq was arrested two years ago and is accused of killing several Shiites, Mohammed Ali said, although he had no details of the charges against the suspect.