Twenty-two people have been killed and at least 59 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.
PM Theresa May said her thoughts were with those affected by “what is being treated by the police as an appalling terrorist attack”.
Paramedics at the scene told the BBC they had treated some of the wounded for “shrapnel-like injuries”.
North West Ambulance Service said it had taken 59 casualties from the explosion to hospitals and treated a number of walking wounded.
The prime minister has suspended Conservative Party general election campaigning and will chair a meeting of the government’s emergency Cobra committee later, in response to the attack.
Mrs 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,” Mrs May said.
Unconfirmed reports from two unnamed US officials suggested the attack was carried out by a suicide bomber.
Labour leader Jeremy Corbyn tweeted: “Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services.”
Greater Manchester metro mayor Andy Burnham said: “My heart goes out to families who have lost loved ones, my admiration to our brave emergency services. A terrible night for our great city.”
The explosion occurred shortly after Ariana Grande left the stage at the arena, the city’s largest indoor venue with a concert capacity of around 18,000.
