Police say Malik Ishaq, head of banned group behind Shia attacks, killed when supporters tried to free him from custody.
Pakistani police have killed the head of the banned group Lashkar-e-Jhangvi as he attempted to escape custody in Punjab province, officers have said.
Malik Ishaq, along with his two sons Usman and Haq Nawaz, and 11 others, were killed in an exchange of fire with police personnel late on Tuesday night, police said. At least six policemen were injured in the encounter.
Al Jazeera's Kamal Hyder, reporting from Islamabad, said: "Malik Ishaq was arrested one week ago along with his sons. He had already spent 14 years in jail and was released on bail in 2011."
Ishaq was on a US list of global "terrorists" and the group he founded has claimed responsibility for the deaths of hundreds of civilians, most of them minority Shia Muslims.
Ishaq and his sons were arrested by the country's Counter-Terrorism Department (CTD) a week ago.
Two high-ranking police officers told the Reuters news agency that Ishaq and his two sons were being moved between detention centres when a group of armed men attacked the convoy and freed them.
A spokesman for the CTD said Ishaq, his two sons, Ghulam Rasool Shah, and two other accused, all from Lashkar-e-Jhangvi, were being taken to Muzaffargarh by the department to aid in the recovery of arms and explosives.
The spokesman said that when the police party was returning after making the recovery, it was attacked by 12 to 15 gunmen who succeeded in freeing Ishaq, his sons and the other accused and fleeing away on motorcycles.
The armed men were met with by officers from a CTD police station who had been informed about the attack on the police party and were travelling on the route that the armed men had taken, the spokesman said.
The officers challenged the armed men, resulting in the encounter in which six police personnel sustained injuries, the spokesman said. They were shifted to the district headquarters hospital.
A large amount of weapons and ammunition was recovered from the armed men and an investigation has been initiated into the events, police said.
All the bodies have been moved to Muzaffargarh District Hospital. The bodies of Ishaq and his sons will undergo a postmortem before being taken to Rahim Yar Khan, where he was based.