A suicide bomber killed 22 people, including children, as an explosion tore through through fans leaving an Ariana Grande concert at the Manchester Arena.
Fifty-nine people were also injured in the blast when the attacker, now named as Salman Abedi, 22, detonated an improvised explosive device shortly after Grande finished her performance on Monday night.
It is the worst terror attack in the UK since 56 people were killed in the 7 July London bombings in 2005.
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- Everything we know about the attack so far
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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 were working to establish if he "was acting alone or as part of a network".
Police were called to reports of an explosion at the Manchester Arena at 10.33pm, shortly after US singer Ariana Grande had finished her performance.
Victims described being thrown by the blast that scattered nuts and bolts across the floor.
More than 240 calls were made to the emergency services, with 60 ambulances flooding the area.
More than 400 police officers were deployed as part of the operation, with a visible presence on the streets of Manchester on Tuesday.
The injured were treated at hospitals across Greater Manchester, and a hotline was set up for those with concerns about loved ones who remain unaccounted for.
Police have appealed for concert-goers and witnesses to provide them with any footage they have from the scene if they believe it can assist the probe.
The Prime Minister has condemned the "appalling" incident and general election campaigning has been suspended.
Grande said she had been left "broken" by the events.
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."
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