At least 47 people have been killed in a bomb blast that apparently targeted policemen offering Zuhr prayers at a mosque in Peshawar's Police Lines area, according to a media report.
Officials said people are still buried under the rubble, BBC reported.
Early unconfirmed reports said a bomber had blown himself up in the mosque.
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Prime Minister Shehbaz Sharif said that "terrorists want to create fear by targeting those who perform the duty of defending Pakistan".
According to Geo News, the outlawed Tehreek-e-Taliban Pakistan (TTP) has taken responsibility for the attack. The Peshawar mosque attack is by far the biggest in the serious of attacks that TTP has been conducting targeting the security forces and their installations.
The group ended a ceasefire in November last year, and violence has been on the rise ever since.
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Peshawar police chief Muhammad Ijaz Khan told local media that between 300 and 400 police officers were in the area at the time of the blast, BBC reported.
In a statement, Sharif said those behind the attack "have nothing to do with Islam", adding, "The entire nation is standing united against the menace of terrorism."
The blast took place at around 1.30 p.m. during afternoon prayers in the north-western city, located near the country's border with Afghanistan.
A video circulating on social media and verified by the BBC showed that half of a wall caved in. The mosque was covered in bricks and debris as people clambered over the rubble to escape.
A rescue operation is continuing inside the mosque and "more bodies are being taken out", Peshawar city Deputy Commissioner Shafiullah Khan said.
"Currently our priority is to save the people buried under the debris," Khan added.