Pakistan's security forces say they have arrested a militant leader wanted in connection with attacks that have killed dozens of minority Shia Muslims.
Asif Chotu (right) and Rashid from police records
Asif Chotu, said to be a senior figure in the Lashkar-e-Jhangvi Sunni Muslim group, was arrested close to the capital, Islamabad, officials said.
Interior Minister Aftab Sherpao refused to confirm or deny the reports.
About 4,000 people have died in Pakistan in the past 25 years of sectarian strife.
The BBC's Aamer Ahmed Khan in Karachi says the arrest of Asif Chotu would be a major coup for the Pakistani security forces.
He says Asif Chotu is credited with reorganising Lashkar-e-Jhangvi after its top figures were either arrested or killed between 2001 and 2003.
The details of Mr Chotu's arrest remain confused.
Some reports say he was detained in a raid on a home near the capital, others that he was arrested when a car was stopped on the road between Rawalpindi and Lahore.
May 2005: 19 killed and nearly 100 wounded in Islamabad mosque attack
March 2005: 43 Shias killed in a bomb blast in Fatehpur, Baluchistan
Oct 2004: Car bomb in Multan kills 40 Sunnis
Oct 2004: 30 killed in a suicide attack on a Sialkot Shia mosque
May 2004: 20 killed in bombing of Shia mosque in Karachi
May 2004: 15 die in Karachi Shia mosque attack
Police have linked Mr Chotu to attacks on Shias in Karachi and Sialkot.
The most deadly was the attack on a Shia mosque in Sialkot in October 2004 that left 30 people dead.
The latest attack police have linked Mr Chotu to is that on a Shia mosque in Karachi on 30 May this year. It sparked a riot that left six people dead.
Police say they found a letter written by Mr Chotu in the possession of one of the arrested mosque attackers.
One security official told the AFP news agency: "He was the most wanted sectarian militant in Pakistan."
He said another Lashkar-e-Jhangvi militant, named Rashid, alias Shahid Satti, was arrested with Mr Chotu.
Lashkar-e-Jhangvi, which was banned in 2001, has been held responsible for many attacks on Shias since its formation in 1996.
Named after hardline cleric, Haq Nawaz Jhangvi, analysts say most of its fighters were trained in Afghanistan at the time of the Taleban regime.
About 80% of Pakistanis are Sunni Muslim and almost 20% Shia.
The schism dates to a 7th century dispute over who should succeed the Prophet Mohammad as leader of the Islamic community.