Twenty two people have been killed and about 50 injured in a suspected terror attack at Manchester Arena.
Police were called to reports of an explosion at the venue at about 22:35 BST on Monday following a pop concert by the US singer Ariana Grande.
The cause of the blast is unknown but North West Counter Terrorism Unit is treating it as a possible terrorist incident.
Greater Manchester Police is working to set up a contact number for relatives.
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.
British Transport Police say the explosion was in the foyer area of Manchester Arena.
The BBC’s Tim Ashburner, who is at the scene, spoke to some volunteer paramedics who treated the injured for “shrapnel-like injuries”.
Shortly after the blast Manchester Victoria station, which is close to the concert venue, 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 confirmed it was not a suspicious item.
‘Screaming and running’
A number of eyewitnesses have described the confusion in the aftermath.
Andy Holey, who had gone to the arena to pick up his wife and daughter who had been at the concert, said: “As I was waiting, 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.
“When I couldn’t find them, I went outside with the police and fire and looked through some of the bodies to try and find my wife and daughter.
“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 their daughters, 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.”