Nineteen people have been killed and more than 50 injured in a suspected terror attack at Manchester Arena.
The blast happened at 22:35 BST on Monday following a pop concert by the US singer Ariana Grande.
Home Secretary Amber Rudd said it was “a barbaric attack, deliberately targeting some of the most vulnerable in our society”.
Relatives are using social media to hunt for missing loved ones, including children, caught up in the incident.
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North West Ambulance Service said it had taken 59 casualties from the explosion to hospitals and treated a number of walking wounded, including for “shrapnel-like injuries”.
Greater Manchester Police has established an emergency telephone number in response to the attack. It is: 0161 856 9400.
In light of the attack, political parties have suspended campaigning ahead of the general election.
The prime minister has announced she will chair a meeting of the government’s emergency Cobra committee at around 09:00 BST.
Theresa May said: “We are working to establish the full details” of what happened in Manchester.
“All our thoughts are with the victims and the families of those who have been affected.”
BBC home affairs correspondent Daniel Sandford said senior counter-terrorism officers were assembling in London and liaising with the Home Office.
Unconfirmed reports from two unnamed US officials suggested the attack was carried out by a suicide bomber.