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.
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 at around 09:00 BST, 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.
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.
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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."
British Transport Police said the explosion was in the arena's foyer, which connects with Victoria train and tram station, a major hub on the northern edge of the city centre.
Shortly after the blast the station was closed and all trains cancelled.
Greater Manchester Police carried out a precautionary controlled explosion in the Cathedral Garden area of the city at about 01:32. The force later confirmed it was not a suspicious item.
The arena 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.
Grande – a 23-year-old American TV teen actress-turned-pop star – has a strong following among teenage girls and children.
The pop star tweeted: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
Pop star's sorrow at concert deaths
In the aftermath of the explosion witnesses spoke about the fear and confusion that gripped those caught up in the events.
Andy Holey, who had gone to the arena to pick up his wife and daughter who had been at the concert, said: "An explosion went off and it threw me about 30ft from one set of doors to the other set of doors.
"When I got up I saw bodies lying on the ground. My first thought was to go into the arena to try to find my family.
"I managed to find them eventually and they're OK.
"It was definitely an explosion and it was some force. It happened near the box office at the entrance to the arena."
Emma Johnson said she and her husband were at the arena to pick up her children, aged 15 and 17.
"It was definitely a bomb. It was definitely in the foyer," she told BBC Radio Manchester.
"We were stood at the top of the stairs and the glass exploded – it was near to where they were selling the merchandise.
"The whole building shook. There was a blast and then a flash of fire afterwards. There were bodies everywhere."
BBC reporter Tom Mullen, who was at scene shortly after the blast, witnessed "sheer panic" among many young people, some with parents or guardians, in the city centre.
In the streets around the arena he saw concertgoers streaming away from the venue in confusion, many of them in tears.- (BBC)
Nineteen people have been killed and more than 50 injured in a suspected terror attack at Manchester Arena.