At least 19 people were killed on Friday in the Pakistani city of Peshawar in a series of explosions targeting a Shia mosque.
Radical Sunni Islamist groups often target mosques frequented by minority Shia, whom they see as infidels.
Police said a group of armed men broke into the mosque, where people were attending Friday prayers, and opened fire, following which three explosions were heard inside the building.
“Police have been called and an operation has started against the terrorists,” said Mian Saeed, police chief of the northwestern city.
The Pakistani Taliban, who are fighting against the state to set up a hardline Sunni theocracy in Pakistan, claimed responsibility.
A witness, Shahid Hussain, told Reuters the worshippers had just finished prayers when five or six men wearing military uniforms broke into the mosque and started shooting.
“We had no idea what was going on. One of the attackers then blew himself up and then there was huge smoke and dust all around,” he said.
The attack came as Pakistan tries to adopt new measures to tackle Islamist militants following a massacre of 134 children on December 16 at an army-run school in Peshawar.