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.
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.
Labour leader Jeremy Corbyn said he was "horrified" by the attack, adding: "Today the whole country will grieve for the people who have lost their lives."
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 explosion occurred shortly after Ariana Grande left the stage at the arena - the city's largest indoor venue with a concert capacity of around 21,000.
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."
In the aftermath of the explosion witnesses spoke about the fear and confusion that gripped those caught up in the events.
Those inside the arena described clothing and mobile phones strewn across the floor as people scrambled for the exits.
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.
Greater Manchester Police has established a help centre for anyone who needs assistance in tracing loved ones. It said help is available at the Etihad Stadium access Gate 11.
Twitter has been flooded with appeals from relatives and friends of missing concertgoers via the hashtag #MissingInManchester.
Facebook also activated a safety check feature so that people can let their family and friends know they are safe.
Within an hour of reports of the incident emerging, people began offering spare rooms and beds to people stranded in the city using the hashtag #RoomForManchester.
Hundreds of tweets offering places to stay were shared and re-tweeted thousands of times.