Greater Manchester Police said 19 people were confirmed dead and roughly 50 were injured by the explosion at Manchester Arena.
"There was a lot of children there without parents".
Ivo Delgado, another witness, recalled hearing one explosion before he saw people running.
Eyewitness Karen Ford told the BBC, "The lights had come up everyone was just getting out and walking towards the stairs, when all of a sudden this huge sound, which sounded like an explosion went off".
"Everyone was screaming and running", Robert Tempkin, 22, told The Times.
Grande said today that she was "broken" and at a loss for words over the deadly terror attack at her concert.
"Words can not express our sorrow for the victims and families harmed in this senseless attack", the team said in a statement on Twitter. Her ruling Conservative Party was preparing to suspend campaigning ahead of the election due to the suspected attack.
Police said they were treating the incident as terrorism.
My thoughts are with all those affected by this barbaric act.
May will hold a crisis response meeting on Tuesday.
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There was no immediate claim of responsibility. Meanwhile, the U.S. official told that the United Kingdom counterparts had identified a male bomber.
Quebec Premier Philippe Couillard, who is now on a trade mission to Israel, told a news conference in Tel Aviv on Tuesday that people must continue to live their lives as normal despite the threats of terrorism.
Islamic State supporters took to social media to celebrate the blast and some encouraged similar attacks elsewhere.
Britain's terrorist threat level has been set at "severe" in recent years indicating an attack is highly likely.
A few hours after the blast, police carried out a controlled explosion in a small park near the venue but said the item turned out to be only abandoned clothes.
"I'm heartbroken over what happened in Manchester tonight Sending love to everyone involved", Harry Styles tweeted.
The victims are being treated at six hospitals across Greater Manchester, Chief Constable Ian Hopkins said.
"I am EXTREMELY proud to be an American", she wrote.
Train services to and from Manchester Victoria Station - located under the Arena - had been cancelled.
"We ran out", Robinson told Reuters. "It was literally seconds after the explosion. I got the teens to run with me". "May it be a comfort to those grieving in Manchester tonight that so many around the world mourn in solidarity with them", Pelosi said.
"It was absolutely terrifying", she said.
Through tears, Campbell said she didn't know how anyone could do this to "innocent children".