About 130 others were injured in the attack on All Saints Church at Kohati Gate area of Peshawar. The first bomber set off his suicide vest as people were emerging from Sunday mass. The second bomber struck within a gap of 30 seconds, said city Commissioner Sahibzada Muhammad Anis.
Over 30 women and at least seven children were among the 78 people killed, officials told the media. The dead included a Muslim policeman who was guarding the church.
Jandullah spokesman Ahmed Marwat told Newsweek: "Until and unless drone strikes are stopped, we will continue to strike wherever we find an opportunity against non-Muslims."
The group had earlier claimed responsibility for killing 10 foreign climbers in Gilgit-Baltistan and for an attack on an Inter-Services Intelligence compound in Sukkur.
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Anis said some 600 to 700 people were inside the church at the time of the attack. People soaked in blood wailed for help as bodies lay strewn in the church's courtyard.
Bomb Disposal Squad chief Shafqat Mahmood said the heads of the bombers had been recovered and sent for forensic analysis. They will be used to prepare sketches, he said.
This was the worst attack in Pakistan's history on the Christian minority, which has not been targeted as frequently as other minorities like Shias or Ahmadis.
The brazen attack sparked protests by Christians across the country. In Peshawar, angry Christians blocked several roads with bodies of the dead and burning tyres. They burned police uniforms too.