Children were among the 22 people killed in a suicide attack after an Ariana Grande concert at Britain's Manchester Arena, police confirmed Tuesday.
Meanwhile, Manchester Police Chief Constable Ian Hopkins identified the suicide bomber responsible for the carnage as 22-year-old Salman Abedi and warned angry residents against any reprisals.
"We understand that feelings are very raw right now and people are looking for answers," he said. "We will not tolerate hate towards any parts of our community."
Police have also arrested a 23-year-old man in Manchester in connection with the attack, said Hopkins, who did not identify him.
Abedi had a British passport, sources told NBC News, but it was not immediately clear where he was born. He was killed when he detonated an improvised explosive device near one of the venue's exits at 10:33 p.m. local time Monday, authorities said.
While Hopkins did not identify any of the dead, 8-year-old Saffie Rose Roussos' death was confirmed by her school.
Some 59 other people were wounded, including some who suffered life-threatening injuries, officials said.
And thousands of grieving Manchester residents were expected to attend a vigil at 6 p.m. local time Tuesday to remember the dead.
ISIS claimed responsibility for the attack but neither US nor British officials have corroborated that.
The pop star's fan base is primarily girls and teenagers, so the venue was packed full of families.
The blast sparked a nightlong search for loved ones as frantic parents tried to locate their children, and groups of friends scattered by the explosion sought each other.
Video shot inside the 21,000-capacity venue showed terrified teenagers screaming as they made their way out amid a sea of pink balloons. Some fans were still wearing the singer and former Nickelodeon TV actress' trademark kitten ears as they fled.
"We have been treating this as a terrorist incident," Greater Manchester Police Chief Constable Ian Hopkins told reporters. "Our priority is to establish whether he was acting alone or as part of a network."
Rohit Kachroo, security editor for NBC News' U.K. partner ITV News, reported that nuts and bolts were spotted in the arena's foyer but police would not comment on whether victims had suffered wounds from shrapnel.
Police said the explosion took place outside the arena catching people as they exited, also triggering chaos inside the venue.U.S. officials said initial reports indicated that some of the injuries might have been caused by a stampede of concertgoers.
Nicola Murray, who was at the concert with her 12-year-old daughter Olivia, told NBC News that she witnessed "a red-orange-looking flash" and heard an "incredibly loud bang."
"Within seconds, as you can imagine, it was chaos and people were panicking and running to try and get out the door," she said. "We were getting swept with the people crushing to get out and I was terrified my daughter would get crushed. At one point, she was getting dragged away from me but I managed to pull her into me and force her in front of me while trying to direct her to the door and down the steps."
Murray, who had traveled to the concert from the Scottish town of Hamilton, said the area outside the venue was "full of people who were completely traumatized — kids and adults alike all crying, screaming, some still running away ... adults in cars who were due to be picking up kids screaming trying to find them."
Alison Pritchard, 34, who works as a waitress, recalled hearing "an almighty explosion behind us."
Her friend Carole Taylor, a 49-year-old teacher, told NBC News she turned around to see "this plume of smoke coming over and all this sort of debris and embers floating from the roof."
"When it exploded, it just rocked your whole body," she said. "It went right through us. People started screaming."
Steve Rotheram, the mayor of Liverpool, told NBC News his two daughters and two nieces were at the show. They described "pandemonium" at the scene with "people climbing over seats to get to exits [and] obviously there were a lot of young girls so there was a lot of screaming and panic," he said.
However, Rotheram's four relatives managed to escape safely.
Hopkins, the police chief, confirmed that children were among the dead and warned of "difficult days ahead."
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British Prime Minister Theresa May said the terrorist set out "to cause maximum carnage and kill and injure indiscriminately."
She added: "This attacks stands out for its appalling, sickening cowardice — deliberately targeting innocent and defenseless young people who should have been enjoying one of the most memorable nights of their lives."
Police said emergency services received 240 calls after the incident and victims were taken to eight hospitals. At least a dozen people aged under 16 were hospitalized.
Jon Rouse, a local health official, said the wounded included "a number of individuals who have very, very serious injuries ... and people who are going to be in hospital for a long time."
Grande, 23, tweeted early Tuesday: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
In a statement, the pop star's manager Scooter Braun thanked first responders "who rushed towards danger to help save lives."
He added: "We ask all of you to hold the victims, their families and all those affected in your hearts and prayers."
Bianca Landrau, the Boston hip-hop star known as Bia, who also performed, tweeted: "Guys we are okay!!!"
And singer Victoria Monét, the opening act, said in a statement that she was safe but "heartbroken that loved ones who came to have the night of their lives ended up losing them."
"I wish I could say that I am OK, but I am not," she wrote, adding: "Please send all your prayers up high for Manchester."
Britain's terrorist threat level stands at "severe," the second-highest rung on a five-point scale, meaning an attack is highly likely.
White House Press Secretary Sean Spicer said President Donald Trump's national security team was keeping him up to date while he is in Israel.
Speaking after meeting with Palestinian leader Mahmoud Abbas in Bethlehemon Tuesday, Trump blamed "evil losers" for the attack, adding that he won't call terrorists "monsters" as "they'd think that's a great name."
He added: "We cannot stand for a moment longer for the slaughter of innocent people."
The Department of Homeland Security stressed that there was no information to indicate a specific credible threat involving U.S. music venues.
Manchester Arena, which opened in 1995, is one of the largest indoor venues in Europe. More than 1 million people fill its seats every year. It is located near the Manchester Victoria transit station.
Local families offered beds for the night to people who had been affected by the explosion, and taxis provided free rides.
A statement issued by Muslim organizations in the Manchester area said the community was "saddened and horrified" by the attack.
It added: "The perpetrators, whoever they may be, must bear the full consequences of their actions."
This was Britain's deadliest terrorist attack since four suicide bombers killed 52 London commuters on three subway trains and a bus in July 2005.
Manchester is located 160 miles northwest of London. It was hit by a huge Irish Republican Army bomb in 1996 that leveled a swath of the city center. More than 200 people were injured in that attack, though no one was killed.