At least 47 people have been killed by a bomb that apparently targeted policeman praying in a mosque in Peshawar, Pakistan.
Officials say people are still buried under the rubble.
Prime Minister Shehbaz Sharif said "terrorists want to create fear by targeting those who perform the duty of defending Pakistan".
No group has said it carried out the attack, but it has been linked to the Pakistan Taliban.
The group ended a ceasefire in November, and violence has been on the rise since.
In December it targeted a police station - like Peshawar, in the north-west of the country - leading to the deaths of 33 militants. Early unconfirmed reports said a bomber had blown himself up in the mosque.
Peshawar police chief Muhammad Ijaz Khan told local media that between 300 and 400 police officials were in the area at the time.
In a statement, PM Sharif said those behind the attack "have nothing to do with Islam". He added: "The entire nation is standing united against the menace of terrorism."
The blast took place around 13:30 (08:30 GMT) during afternoon prayers in the north-western city, 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 people buried under the debris," Mr Khan added.
Mohammad Asim, a spokesman for the city's Lady Reading Hospital, said that some of those injured were in a critical condition.
"It's an emergency situation," Mr Asim added.
Lady Reading Hospital has declared an emergency and told the BBC they were still receiving those injured.
The prime minister travelled to Peshawar on an emergency visit, where he will be briefed by local officials and visit those wounded by the blast.
Last March, Peshawar was the target of another bombing, which killed dozens in a Shia mosque.
In the capital, Islamabad Police issued a security high-alert and said security at all entry and exit points to the city had been increased.