Pakistani Minorities Minister Shahbaz Bhatti has been shot dead by gunmen who ambushed his car in broad daylight in the capital Islamabad.
He was travelling to work through a residential district when his vehicle was sprayed with bullets, police said.
Mr Bhatti, the cabinet's only Christian minister, had received death threats for urging reform to blasphemy laws.
In January, Punjab Governor Salman Taseer, who had also opposed the law, was shot dead by one of his bodyguards.
The blasphemy law carries a death sentence for anyone who insults Islam. Critics say it has been used to persecute minority faiths.
'No security escort'
The Vatican condemned the murder of the Catholic politician as an "unspeakable" act of violence.
Mr Bhatti, 42, a leader of the ruling Pakistan People's Party (PPP), had just left his mother's home in a suburb of the capital when several gunmen surrounded his vehicle and sprayed it with bullets, say witnesses.
One witness, Gulam Rahim, told AP news agency that two of the attackers had opened the door and tried to pull Mr Bhatti out, while another man fired a Kalashnikov into the car.
He was taken to the nearby Shifa hospital, but was dead on arrival.
The gunmen, who were wearing shawls, escaped in a white Suzuki car, according to witnesses.
Police chief Wajid Durrani told reporters that the minister was not accompanied by his guards when the attack happened, although he said Mr Bhatti had been provided with a security detail.
Pamphlets by al-Qaeda and Tehrik-i-Taliban Punjab, a branch of the Taliban in Pakistan's most populous province, were found at the ambush site.
Tehrik-i-Taliban told BBC Urdu they carried out the attack.
"This man was a known blasphemer of the Prophet [Muhammad]," said the group's deputy spokesman Ahsanullah Ahsan.
"We will continue to target all those who speak against the law which punishes those who insult the prophet. Their fate will be the same."
Vendettas
Security has been stepped up on all main roads in Islamabad.
In January, Mr Bhatti told the BBC he would defy death threats he had received from Islamist militants for his efforts to reform the blasphemy law.
A government spokesman condemned the assassination.
"This is a concerted campaign to slaughter every liberal, progressive and humanist voice in Pakistan," Farahnaz Ispahani, an aide to President Asif Ali Zardari, told AP news agency.
"The time has come for the federal government and provincial governments to speak out and to take a strong stand against these murderers to save the very essence of Pakistan."
Governor Taseer was shot dead on 4 January, also in Islamabad, by one of his own police bodyguards. The killer has been feted by many in the country as a hero.
The governor had backed a private member's bill in parliament by Sherry Rehman, a female MP, to amend the blasphemy law in an attempt to make miscarriages of justice less likely and remove its death penalty.
But in the face of strident popular opposition, the federal government said it would not support the proposed reforms.
Ms Rehman said last month she was receiving death threats every half hour by e-mail and telephone.
Christians, who make up an estimated 1.5% of Pakistan's 185 million population, were left reeling by Mr Bhatti's death.
"We have been orphaned today!" Rehman Masih, a Christian resident of Islamabad, told AP news agency. "Now who will fight for our rights?"
Pakistan's blasphemy law has been in the spotlight since a Christian, Asia Bibi, was sentenced to hang in Punjab last November.
She denies claims she insulted the Prophet Muhammad during a row with Muslim women villagers about sharing water.
Although no-one convicted under the law has been executed, more than 30 accused have been killed by lynch mobs.
Critics say that convictions under the law hinge on witness testimony, which is often linked to grudges.
