Pakistani police say they have shot dead Malik Ishaq, the leader of banned Sunni militant group Lashkar-e-Jhangvi.
Ishaq, who had been detained last week, was in a police convoy in Punjab province when supporters opened fire in an attempt to free him, said police.
They said Ishaq was killed in a gun fight along with his two sons and 11 militants in Muzaffargarh district.
Lashkar-e-Jhangvi (LeJ) has been behind some of the most violent attacks on Shia Muslims in recent years.
Punjab Home Minister Shuja Khanzada confirmed the news to AFP news agency, while a doctor at Muzaffargarh District Hospital said 14 bodies had been received there.
Six police officers were also reported injured but correspondents say the extent of their injuries is unclear.
The exact circumstances of the killing remain unclear.
Ishaq, his two sons and other alleged members of LeJ were taken into custody by the Counter Terrorism Department several days ago in connection with eight recent sectarian killings.
Late on Tuesday they were being led to Shahwala, on the outskirts of Muzaffargarh city, “where they (prisoners) said they had hidden a cache of arms and ammo”, police said.
Police claim the group was attacked by 10 or 12 armed men in an attempt to free the prisoners. All of them were killed in the ensuing gunfight, they said.
Ishaq was in police custody for more than 13 years on over 70 charges of sectarian killings. He walked free in December 2014 after a decision by Lahore High Court.
LeJ was banned in Pakistan in 2001 and designated a terrorist group by the US in 2003. It has claimed the killings of hundreds of mainly Shia civilians in Pakistan.