Twenty-two people have been killed and at least 59 people injured in a terrorist bombing during an Ariana Grande concert at Manchester Arena.
Children are known to be among the dead.
It is the worst terror attack in the UK since 56 people were killed in the 7 July London bombings in 2005.
Footage showed thousands of people fleeing the arena in a panic after an explosion ripped through the foyer as people milled out of the arena at 10.33pm.
Eyewitnesses said the explosion was heard outside the auditorium after the artist had finished her show and left the stage.
Casualties were taken to five hospitals throughout the city and victims described being thrown by the blast that scattered nuts and bolts across the floor.
The Prime Minister has condemned the "appalling" attack and General Election campaigning has been suspended.
Ariana Grande said on Twitter she was "broken" by the devastating attack.
Announcing that the death toll had risen overnight, Greater Manchester Police chief constable Ian Hopkins said: "What I can confirm is that there are children among the deceased."
He said: "This has been the most horrific incident we have had to face in Greater Manchester and one that we all hoped we would never see.
"Families and many young people were out to enjoy a concert at the Manchester Arena and have lost their lives."
Mr Hopkins said investigators believed the attack was conducted by one man who died at the scene, although detectives are working to establish if he "was acting alone or as part of a network".
Andy Burnham, the Greater Manchester metro mayor, said the attack was an "evil act".
Speaking on Tuesday morning the former Labour MP said: "It is hard to believe what has happened here in the last few hours, and to put into words the shock, anger, and hurt that we feel today.
"These were children, young people and their families that those responsible chose to terrorise and kill.
"This was an evil act."
He added: "We are grieving today, but we are strong. Today it will be business as usual, as far as possible in our great city."
Concert goer, Josh Elliot, told the BBC there was pandemonium and confusion as people tried to flee the scene, some reportedly covered in blood and many in tears.
He said: "She finished her last song .. and we heard like a really basey bang, and all I can remember is just everyone ... was just running out... people were covered in blood, some people were having a fight, it was quite scary."
One fan leaving the concert described how the bangs ripped through the arena, which has a capacity of 21,000. It is not known how many people were attending the concert, but the majority of the American pop star's fans are young females in their teens and twenties.
"The lights came on after the gig and people were starting to leave. As I turned to the left there was an explosion. It was about 40ft behind us near one of the exits. We just thought it was people messing about then it happened again. Another explosion sounded.
“Then we saw the smoke. Everyone just fled. Some people were injured. We saw blood on people when we got outside. People were just running all over the place.
“People have said it was a speaker or balloons but it wasn’t. It was explosions.”
Majid Khan, 22, said: "I and my sister, along with a lot of others were seeing Ariana Grande perform at Manchester Arena, and we were all exiting the venue when around 10.40-10,45pm-ish, a huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena.
"It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit Trinity Way and that was blocked so everyone was just running to any exit they could find as quickly as they could.
"Everyone was in a huge state of panic, calling each other as some had gone to the toilet whilst this had gone off, so it was just extremely disturbing for everyone there."
He said there were "a good 20 to 30 people lying injured" close to the box office just outside the main arena.
"Whoever's done it has done it on a kids' concert," he said.
Another eyewitness said he was thrown "30 feet in the air" by the force of the explosion which was reported to have shaken the entire building.
Another witness, Karen who was at the concert with her husband and children described the pandemonium as thousands tried to escape the venue after the explosion sound was heard.
"It was just chaos... nobody was organising anybody to do anything."
She added that she saw a woman in a wheelchair being pushed over as people desperately tried to get out after someone shouted there had been a bomb.
Liverpool City Metro Mayor Steve Rotheram said on Twitter that his two daughters had been at the concert but had made it home safely. He thanked the taxi driver who had assisted them, and offered his sympathies to other parents waiting anxiously for news of their children.
Police warned people to stay away from the arena while investigations continued before they put out a statement confirming the number of fatalities shortly after 1am.
Police confirmed a bomb disposal squad carried out a controlled explosion of a suspect device around 1am, although this turned out to be benign.
Prime Minister Theresa May said in a statement: "We are working to establish the full details of what is being treated by the police as an appalling terrorist attack.
"All our thoughts are with the victims and the families of those who have been affected."
Other politicians including Labour leader Jeremy Corbyn, Greater Manchester Mayor Andy Burnham and MP Lucy Powell expressed their shock and offered condolences on Twitter.
Manchester City football club tweeted: "It's with great sadness we hear of the terrible events at the Arena. Our hearts go out to all affected and to our city's emergency services.''
Offers of help and shelter for people stranded in the city began flooding in on social media.
Greater Manchester Chief Constable Ian Hopkins said at a press conference in the early hours of Tuesday morning that the force is working with other officials to investigate the circumstances of the explosion.
He urged people to avoid the area while the investigation continues.
He said: "We are currently treating this as a terrorist incident until we know otherwise. We are working closely with the national counter-terrorism policing network and UK intelligence partners.
"This is clearly a concerning time for people but we are doing all we can, working with local and national agencies to support those affected and gather information about what happened last night."
A police hotline to assist those concerned about loved ones who may have been caught up in the explosion should call 0161 856 9400.
Anyone with information which may assist the investigation is asked to call the anti-terrorist hotline on 0800 789321.
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