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At least 61 people were killed and dozens of others were wounded in a suicide bomb attack on a mosque inside a police compound in north-west Pakistan on Monday.
The Pakistani Taliban claimed responsibility for the blast in Peshawar, which caused sections of the building to collapse onto worshippers, and hospitals said as many as 150 people had been wounded.
Officials were investigating how the bomber was able to sneak into the mosque inside a police headquarters housing sensitive intelligence and counter-terrorism offices.
The attack is thought to be the deadliest Taliban strike on Pakistan's state in nine years.
The region of Pakistan bordering Afghanistan has seen an increase in bombings since the Pakistani Taliban's Afghan allies took control of Kabul.
Policemen attempted to dig their colleagues out of the rubble after a wall and sections of the roof collapsed. Shahid Ali, a policeman who survived, said the explosion took place seconds after the imam started prayers.
"I saw black smoke rising to the sky. I ran out to save my life," the 47-year-old told AFP. "The screams of the people are still echoing in my mind," he added. "People were screaming for help."
In the aftermath of the blast, rescuers dug through fallen masonry to find their colleagues. Muhammad Ijaz Khan, Peshawar police chief, estimated between 300 and 400 officers usually attended prayers at the mosque.
He said: "Many policemen are buried under the rubble. Efforts are being made to get them out safely."
Many of the wounded were listed as critical, suggesting the death toll would rise further.
A leader of the Pakistani Taliban said the bombing had been carried out to avenge the killing six months ago of Omar Khalid Khorasani, one of the group's founders.
The militant group, also known as Tehreek-e-Taliban Pakistan or TTP, has waged an insurgency over the past 15 years seeking the overthrow of the government and stricter enforcement of Islamic laws.
The group is separate from the Afghan Taliban, but retains close ties and many similar views.
At the height of the group's insurgency, Peshawar suffered regular suicide bombings. Successful operations by the Pakistani army eventually pushed the militants into Afghanistan over the last decade.
There they were given a haven by their Afghan allies. Observers say the 2021 victory of the Afghan Taliban over the Nato-backed Afghan government had appeared to embolden the Pakistani militants.
The first 12 months of Taliban rule in Kabul coincided with a 50 per cent surge in militant attacks inside Pakistan, focused in the western border provinces, according to the Pak Institute for Peace Studies (PIPS).
Pakistan's military has unsuccessfully attempted to get the new Afghan Taliban regime to exert pressure on the militants to either give themselves up, or stop attacks. A number of brief truces have ended in failure.
Pakistan was put on high alert after the blast, with checkpoints reinforced and extra security forces deployed. In the capital, Islamabad, snipers were positioned on buildings and at city entrance points.
Shehbaz Sharif, the prime minister, visited the wounded in hospital and vowed "stern action" against the culprits. He called on his party workers to give blood to save the wounded.
"The sheer scale of the human tragedy is unimaginable," Mr Sharif said. "This is no less than an attack on Pakistan. The nation is overwhelmed by a deep sense of grief. I have no doubt terrorism is our foremost national security challenge."
Imran Khan, the former prime minister, condemned the attack.
He said: “It is imperative we improve our intelligence gathering and properly equip our police forces to combat the growing threat of terrorism.”
Observers fear Pakistan is spiralling into an economic crisis, and the government is urgently seeking a crucial instalment of £900m from the International Monetary Fund, part of a £4.85bn bailout package that has been on hold for months.