Update May 23, 8:30 a.m.:
ISIS released a statement claiming responsibility for the suicide bombing at an Ariana Grande concert on Monday night.
The statement from ISIS said in Arabic that "a soldier of the caliphate placed explosives among a gathering of crusaders, meaning Christians, at the Manchester Arena," ABC News reported.
Prime Minister Theresa May called a sickening act targeting children and young people.
May said police believed they knew the identity of the bomber and police then said a 23-year-old man had been arrested in connection with the attack carried out late on Monday evening as people began leaving a concert given by Ariana Grande, a U.S. singer who attracts a large number of young and teenage fans.
"All acts of terrorism are cowardly...but this attack stands out for its appalling sickening cowardice, deliberately targeting innocent, defenseless children and young people who should have been enjoying one of the most memorable nights of their lives," May said outside her Downing Street office in London.
"The attempt to divide us met countless acts of kindness that brought people closer together."
A man has been identified as the probable suicide bomber who killed 22 in a "serious incident" amid reports of an explosion during an Ariana Grande concert on Monday. At least 50 people were reported injured, authorities said.
Officials have not released the name of the man suspected to be the bomber.
"We are currently treating this as a terror incident until we have further information," Manchester's chief constable said during a news conference. He added that the public will be updated when "we have a clearer picture."
Witnesses reported hearing two loud bangs around 10:35 p.m. local time as Ariana Grande finished her performance on Monday with her song "Dangerous Woman." A CNN correspondent reported at least one blast occurred in a hallway outside of the arena – in a public area near where parents were already picking up their children – that resulted in bedlam as concertgoers attempted to flee the venue. Some concert attendees thought the blast was part of the concert.
Shortly after the event, an emotional witness told CNN that she saw 20 or 30 bodies laying on the floor, but at the time, the number of deaths had not been confirmed.
Some deaths and injuries could have been caused by a stampede, NBC tweeted.
"We were making our way out and when we were right by the door, there was a massive explosion and everybody was screaming," Catherine Macfarlane, who was at the concert, told Reuters. "It was a huge explosion – you could feel it in your chest. It was chaotic. Everybody was running and screaming and just trying to get out."
Police are treating the incident as a terror attack until they know otherwise, but they are not confirming that the attack was indeed terror-related. Witnesses reported that many children were at the U.S. pop star's concert. Grande's concerts tend to attract young girls between the ages of 11 and 15.
If confirmed as a terrorism incident, it would be the deadliest attack in Britain by militants since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005. The blast also came two and half weeks ahead of an election in which Prime Minister Theresa May is predicted by opinion polls to win a large majority.
Family members are posting photos of loved ones on Twitter in the hopes that someone has been in contact and knows if they are OK.
Videos on social media show people screaming and running from the venue.
Concertgoer Hannah Dane told the Guardian there was “quite a loud explosion heard from inside the Manchester Arena and it shook, then everyone screamed and tried to get out."
“As we got outside, lots of police came racing towards the area, and the whole of the Victoria train station was surrounded by police,” she added. People were “screaming and crying everywhere, shouting that there’s a bomb, and also people were saying there’s a shooter."
Latest statement on incident at Manchester Arena pic.twitter.com/BEpLOan3dY— G M Police (@gmpolice) May 23, 2017
Police statement on incident at Manchester Arena pic.twitter.com/gaKASukx9a— G M Police (@gmpolice) May 22, 2017
Police blocked off roads and warned people to stay away from the arena.
"Emergency services responding to serious incident at Manchester Arena," Greater Manchester Police said. "Avoid the area."
Officials announced a planned "controlled detonation" of a device discovered about a block away from the arena, but tweeted shortly after that the object thought to be a device was abandoned clothing.
Police are asking the "walking wounded" to identify themselves as paramedics are on the scene. Britain’s ITV Granada, the commercial Independent Television Network station for Manchester, reported people covered in blood fleeing from the arena.
Manchester Arena, the largest indoor arena in Europe, opened in 1995 and has a capacity for 21,000 people, according to its website. It is a popular concert and sporting venue.
Britain is on its second-highest alert level of "severe," meaning an attack by militants is considered highly likely.
British counter-terrorism police have said they are making on average an arrest every day in connection with suspected terrorism.
Two U.S. officials who spoke on condition of anonymity said that initial signs pointed to a suicide bomber as being responsible for the blast. "The choice of venue, the timing and the mode of attack all suggest this was terrorism," said a U.S. counter terrorism official who also spoke on condition of anonymity.
This attack calls to mind the December 2015 attack in Paris at an Eagles of Death Metal concert. The Islamic State claimed responsibility for that attack, which killed more than 100 people. It was the worst such attack in Europe since the Madrid train bombings of 2004, in which 191 died.
In March, a British-born convert to Islam plowed a car into pedestrians on London's Westminster Bridge, killing four people, before stabbing to death a police officer who was on the grounds of parliament. He was shot dead at the scene.
More than 50 were killed at an Orlando nightclub in June 2016 during a shooting by an Islamic State supporter.
A spokesman for Grande's record label said that the 23-year-old singer was "OK."
Reuters contributed to this report.