Screaming fans, many of them teenagers, fled the venue in panic after the bomb blast, which came at the end of a concert by US star Ariana Grande in the northern English city late on Monday.
Police said the attacker was believed to be "carrying an improvised explosive device which he detonated causing this atrocity" and had himself died at the scene, but gave no further details about him.
One victim reported seeing nuts and bolts that could have been packed into a bomb and said he was injured by shrapnel, others saw glass after the blast smashed windows at the venue.
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The blast occurred in the foyer of the indoor arena, a covered area which links the auditorium to Victoria Station, a train and tram hub. Witnesses reported being near the arena's ticket machines and merchandise stores, as chaos ensued inside the concert hall.
"The arena was scarily still for five or six seconds, which felt like a lot longer, and then everybody just ran everywhere," Kennedy Hill, a teenager at the concert, told AFP.
"There were fathers carrying their little girls in tears," said Sebastian Diaz, a 19-year-old from Newcastle.
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.
Greater Manchester Police chief Ian Hopkins said Tuesday there was an unspecified number of children among the 22 dead, while 59 people were also injured. Police believe the blast was carried out by one man and are trying to find out whether he was acting alone.
IS claims responsibility
The Islamic State group claimed responsibility for the bombing.
The group said in a statement published on its social media channels that "one of the caliphate's soldiers placed bombs among the crowds," and it threatened more attacks.
The group's self-styled news agency Amaq separately claimed "a security squad" carried out the attack.
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.
It also revived memories of 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 militant group.
Condemnations
British Prime Minister Theresa May condemned the "appalling terrorist attack" and suspended her campaign for the general election on June 8 along with chief opposition leader Jeremy Corbyn.
US President Donald Trump led condolences from political leaders across the globe, as well as stars from the world of music and football.
Trump, speaking during a visit to Bethlehem, said "evil losers" were behind the attack.
Russian President Vladimir Putin condemned a "cynical, inhuman crime" and offered to boost anti-terrorism cooperation with Britain, while German Chancellor Angela Merkel and French President Emmanuel Macron also expressed condolences.
Home Secretary Amber Rudd described the attack as 'barbaric' and "deliberately targeting some of the most vulnerable in our society -- young people, children out at a pop concert."
"The great city of Manchester has been affected by terrorism before. Its spirit was not bowed," said Rudd.
Britain's third biggest city was hit in 1996 by a massive car bomb planted at a shopping centre by the Irish Republican Army (IRA) which wounded more than 200 people.
May chaired an emergency ministerial meeting at around 0800 GMT.
Grande has reportedly suspended her world tour following the attack.
"Broken. From the bottom of my heart, I am so, so sorry. I don't have words," the 23-year-old, who is popular with teens and pre-teens, wrote on Twitter.
https://twitter.com/ArianaGrande/status/866849021519966208?ref_src=twsrc%5Egoogle%7Ctwcamp%5Eserp%7Ctwgr%5Etweet
Police were called to the scene at the Manchester Arena, on the edge of the city centre, at 2133 GMT.
"A lot of these people at the concert were small children and teenagers like my daughter. These were Christmas presents for the majority of people. What should have been a happy occasion has ended like this, it is just tragic," witness Stephanie Hill told AFP.
Another witness, Gary Walker, told BBC radio he was hit by shrapnel in his foot and his wife sustained a stomach wound as they waited for their daughters. "Someone came through the doors, then bang," he said.
The injured were being treated in eight hospitals across the city.
Facebook gave users in Manchester the option of marking themselves as safe following the explosion.
In a city famed globally for its musical traditions and football teams, showbusiness stars and teams joined in to express their shock at the carnage.
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"We are deeply shocked by last night's terrible events at the Manchester Arena. Our thoughts and prayers go out to all those affected," said Manchester United, who are in Stockholm ahead of Wednesday's Europa League final against Ajax.
Manchester City's captain Vincent Kompany wrote on Twitter: "I would like to express my deepest condolences to the families of the victims. It's a sad day for the great city of Manchester. #Unity".
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."
Broken hearted for the families tonight. Broken hearted for Ari. Broken hearted for the state of this world. 😔
— KATY PERRY (@katyperry) May 23, 2017
https://twitter.com/taylorswift13/status/866824457050099712
In Manchester, residents tweeted with the hashtag #RoomforManchester to offer a place to stay and there were reports of taxis taking passengers for free.
Concerned relatives used the hashtag #MissinginManchester to locate loved ones.
Pakistan condemns Manchester blast
Pakistani Prime Minister Nawaz Sharif also strongly condemned the blast in Manchester. Foreign Office spokesperson Nafees Zakaria while speaking to Radio Pakistan said, "Pakistan, itself has been victim of terrorism and denounces terrorism in all forms and manifestations."
PM Nawaz Sharif has strongly condemned the explosion that occurred in #Manchester, resulting in death of 22 persons and injuring many others
— Ministry of Foreign Affairs - Pakistan (@ForeignOfficePk) May 23, 2017
Pakistan stands in solidarity with the Govt & ppl of UK against all forms of terrorism - PM Nawaz
— Ministry of Foreign Affairs - Pakistan (@ForeignOfficePk) May 23, 2017
He said people of Pakistan, including personnel of security and armed forces, have rendered great sacrifices in the war against terror.
"Pakistan has been in the forefront of all counter-terrorism campaigns," the FO spokesperson added.
When asked if any Pakistanis in Manchester had been harmed, he stated no information has been received thus far. He, however, said the country's Counsel General has been asked to get complete information in this regard.
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