By Agence France-Presse
Screaming fans, many of them teens, fled the venue in panic after the bomb blast at the end of the concert in the northern English city of Manchester on Monday evening.
"There was heat on my neck and when I looked up there were bodies everywhere", Elena Semino, who was waiting for her 17-year-old daughter, told the Guardian newspaper.
Semino, who was herself injured, said she had been standing by the ticket office of the 21,000-capacity indoor Manchester Arena when the explosion went off.
"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena", said Majid Khan, 22, who was at the show with his sister.
Ambulances and bomb disposal teams rushed to the venue, as family members frantically searched for their loved ones, and residents opened their doors to stranded concert-goers after trains were cancelled.
There were children among the 22 killed in the attack, while 59 people were injured, Greater Manchester Police chief Ian Hopkins said early Tuesday.
British Prime Minister Theresa May condemned the "appalling terrorist attack" and suspended her campaign ahead of a general election on June 8 along with chief opposition leader Jeremy Corbyn.
"All our thoughts are with the victims and the families of those who have been affected," said May, who was due to chair an emergency ministerial meeting at around 0800 GMT on Tuesday.
Grande, who was due to give a concert in London later on Tuesday, said she was "broken" in a tweet.
"Broken. From the bottom of my heart, I am so, so sorry. I don't have words," wrote the 23-year-old, who is popular with teens and pre-teens.
The attack was the deadliest in Britain since July 7, 2005 when four suicide bombers inspired by Al-Qaeda attacked London's transport system during rush hour, killing 52 people and wounding 700 more.
The Manchester blast recalled the November 2015 attack at the Bataclan concert hall in Paris in which armed men wearing explosive belts stormed in and killed 90 people. That attack was claimed by the Islamic State jihadist group.
'A massive flash'
Police were called to the scene at the Manchester Arena concert and sports venue at 2133 GMT.
Manchester Arena said the blast "took place outside the venue in a public space", while police said it was "within the foyer area of the stadium."
The foyer connects the auditorium with Victoria train and tram station, a major transport hub on the northern edge of the city centre.
One witness, Gary Walker, told BBC Radio 5 Live he was hit by shrapnel in his foot and his wife sustained a stomach wound as they waited for their daughters.
"We heard the last song go and then suddenly there was a massive flash and then a bang and smoke," he said.
Isabel Hodgins, an actress who had been at the concert, told Sky News: "Everybody was panicking, there was pushing up the stairs.
"The corridor was full, it smelled of burning, there was quite a lot of smoke as we were leaving.
"It's just shocking and we just feel very shaken up. We're just lucky to have gotten away safely," she said.
Calvin Welsford, 18, from Bristol told the BBC: "It almost sounded like a gunshot".
"I looked around and people were just spilling down, heading out of the building".
"I was actually having an asthma attack. It was sheer panic," he said.
US 'ready to assist'
Hopkins said investigators were "working closely with the national counter terrorism policing network and UK intelligence partners".
The US Department of Homeland Security, Britain's biggest intelligence partner, said it was "closely monitoring" the situation.
"We are working with our foreign counterparts to obtain additional information about the cause of the reported explosion as well as the extent of injuries and fatalities," the department said in a statement.
Tributes poured in for Manchester from around the world.
Canadian Prime Minister Justin Trudeau said Canadians were "shocked by the news of the horrific attack," while EU president Donald Tusk wrote: "My heart is in Manchester this night."
French President Emmanuel Macron said he was "horrified" by the attack.
Pop world rallies
The pop world also rallied, with Katy Perry tweeting: "Praying for everyone at Ariana Grande's show".
US pop princess Taylor Swift, a friend of Grande, wrote: "My thoughts, prayers and tears for all those affected by the Manchester tragedy tonight".
In Manchester, residents tweeted with the hashtag #RoomforManchester to offer a place to stay and there were reports of taxis taking passengers for free.
"We have a spare double bed and two sofas available if anybody needs a place tonight," tweeted @iamjesyrae.
Concerned relatives used the hashtag #MissinginManchester to locate loved ones.
Train services to and from Manchester Victoria Station -- located under the Arena -- were cancelled.
"Disruption is expected to continue until the end of the day," National Rail said in a statement.