At least 59 people have been killed by a suicide bombing that
apparently targeted policemen praying in a mosque in Peshawar, Pakistan.
The mosque is within the tightly guarded police headquarters area.
"Terrorists want to create fear by targeting those who perform the duty of defending Pakistan," Prime Minister Shehbaz Sharif said.
The Pakistani Taliban denied involvement after an initial claim by one of its commanders.
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.
A hospital spokesman told the BBC the death toll stood at 59, while 157 people had been injured.
Between 300 and 400 police officers were in the area at the time, Peshawar police chief Muhammad Ijaz Khan told local media.
The mosque is in one of the most heavily controlled areas of the city, which includes police headquarters and intelligence and counter-terrorism bureaus.
Mr Sharif said those behind the attack had "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 shows that half of a wall caved in. The mosque was covered in bricks and debris as people clambered over the rubble to escape.
Hours after the blast, BBC News witnessed a facility full of the injured, many still wearing their police uniforms.
Some were covered in burns cream, their skin red with burns from the explosion. Others have broken bones from being hit by falling rubble.
One man said he still could not hear because of the sound of the blast. Another man said he had been rescued after being trapped under the rubble for almost an hour.
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.
UN Secretary General António Guterres condemned the attack, with his spokeswoman saying: "It is particularly abhorrent that such an attack occurred at a place of worship."
The attack on the mosque took place at the start of a key week for Pakistani diplomacy.
On Monday, the president of the United Arab Emirates, Mohamed bin Zayed Al Nahyan, was due to visit Islamabad - although the trip was cancelled at the last minute because of bad weather.
On Tuesday, an International Monetary Fund delegation is meant to visit Pakistan as part of the process to unlock a bailout loan to prevent the country from defaulting.
Last March, Peshawar was the target of another bombing, which killed dozens in a Shia Muslim mosque in the majority Sunni Muslim country.
In the capital Islamabad, police issued a high alert and said security at all entry and exit points to the city had been increased.