At least 22 people have died and 50 more were injured after a terrorist bombing tore through an Ariana Grande concert in Manchester, police have said.
Chief Constable Ian Hopkins said the sole attacker died in the explosion.
Mr Hopkins said the lone male attacker, who died in the blast, was carrying an improvised explosive device which he detonated.
He said there are children among the people killed in the attack.
Mr Hopkins 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.
"Our thoughts are with those 22 victims that we now know have died, the 59 people who have been injured and their loved ones.
"We continue to do all we can to support them."
Mr Hopkins said a "fast-moving investigation" had established the attack was conducted by one man, although detectives are working to establish if he was "was acting alone or as part of a network".
"The attacker, I can confirm, died at the arena.
"We believe the attacker was carrying an improvised explosive device which he detonated, causing this atrocity."
Witnesses reported hearing a "huge bomb-like bang" at around 10.30pm on Monday, as fans were leaving the arena shortly after a show by Ariana Grande finished, and described glass and metal nuts on the floor.
The Prime Minister has condemned the "appalling" incident and General Election campaigning has been suspended.
Downing Street said a meeting of the Government's emergency Cobra committee is expected to take place at around 9am on Tuesday morning chaired by Theresa May.
It is the worst terror attack in the UK since 56 people were killed in the 7 July London bombings in 2005.
Grande, the US singer who finished performing minutes before the blast, said she had been left "broken" by the events.
Her management team, SB Projects, praised Manchester's emergency services.
They said: "Words cannot express our sorrow for the victims and families harmed in this senseless attack.
"We mourn the lives of children and loved ones taken by this cowardly act.
"We are thankful for the selfless service tonight of Manchester's first responders who rushed toward danger to help save lives.
"We ask all of you to hold the victims, their families, and all those affected in your hearts and prayers."
Sir Richard Leese, leader of Manchester City Council, said: "This is an absolutely horrifying incident and our thoughts are with everyone affected, especially those who have lost loved ones or been injured and traumatised.
"If it is confirmed this was a terrorist attack it is a monstrous act but also a deeply futile one.
"Manchester is a proud and strong city and we will not allow those who seek to sow fear and division to achieve their aims."
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