The influential governor of Pakistan's Punjab province, Salman Taseer, has died after being shot by one of his bodyguards in the capital, Islamabad.
Mr Taseer, a senior member of the Pakistan People's Party, was shot when getting into his car at a market.
Interior Minister Rehman Malik said the guard had told police that he killed Mr Taseer because of the governor's opposition to Pakistan's blasphemy law.
Prime Minister Yousuf Raza Gilani has declared three days of mourning.
He also appealed for calm and ordered an immediate inquiry.
It is the most high profile assassination in Pakistan since the killing of former Prime Minister Benazir Bhutto, the PPP's leader, three years ago.
'Blasphemer'
Mr Taseer, 66, was shot several times at close range by his Elite Force guard as he got into his car at the Kohsar Market, a shopping centre in Islamabad popular with Westerners and wealthy Pakistanis, Mr Malik said.
"The governor fell down and the man who fired at him threw down his gun and raised both hands," Ali Imran, a witness, told the Reuters news agency.
Unconfirmed reports say up to five other people were wounded when Mr Taseer's other bodyguards opened fire following the attack.
It is believed Mr Taseer had been returning to his car after meeting a friend for lunch at a nearby restaurant. He had previously been to the presidential palace, the Senate and the interior ministry.
At a news conference in the capital, Mr Malik said: "The police guard who killed him says he did this because Mr Taseer recently defended the proposed amendments to the blasphemy law."
"This is what he told the police after surrendering himself."
"But we are investigating to find out whether it was his individual act or whether someone else was also behind it," he added.
Mr Taseer made headlines recently by appealing for the pardon of a Christian woman, Asia Bibi, who had been sentenced to death for blasphemy.
Mr Malik later identified the suspect as Malik Mumtaz Hussein Qadri, who he said had escorted the governor from Rawalpindi on Tuesday as he had done on five to six previous occasions.
"Salman Taseer is a blasphemer and this is the punishment for a blasphemer," Mr Qadri said in comments broadcast on Dunya television.
The interior minister said Mr Taseer's Elite Force security detail was provided by the Punjab government, and that its members had been thoroughly screened. However, they have all been arrested as part of the investigation.
The BBC's Aleem Maqbool in Islamabad says Mr Taseer, a close associate of President Asif Ali Zardari, was one of Pakistan's most important political figures and his death will further add to instability in the country.
The PPP-led government is facing a crisis that erupted after a coalition partner quit. Mr Taseer had insisted it would survive.
