Around 600 worshipers had gathered at All Saints Church, where many members of the congregation were participating in a food giveaway on the front lawn, Dawn News reported.
"As soon as the service finished and the food was being distributed, all of a sudden we heard one explosion, followed by another," said witness Azim Ghori.
There was no immediate claim of responsibility from the Taliban or other militant groups for what The New York Times called the worst attack on Pakistan's Christian minority in years.
The incident was immediately condemned by Prime Minister Nawaz Sharif and by the Pakistan Ulema Council, the nation's largest clerical body, which issued a statement saying the group was "standing with our Christian brothers in this tragedy."
Capital City Police Chief Mohammad Ali Babakhel and Police Superintendent Ismail Tariq confirmed there were two bombers, one inside and one outside the church.
Tariq said the two attackers opened fire at security guards, before entering the church, killing one and injuring another.
Inspector Gen. Shafqat Malik of the Bomb Disposal Squad said the remains of the two attackers were recovered and sent for forensic tests.
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