Manchester, England (CNN) Monday's attack outside an Ariana Grande concert at Manchester Arena killed at least 22 people, including children, and was carried out by a lone suspect carrying a bomb, Manchester Police said.

"The attacker, I can confirm, died at the arena. We believe the attacker was carrying an improvised explosive device, which he detonated, causing this atrocity," said Chief Constable Ian Hopkins.

While police believe the suspect was acting alone, investigations are ongoing to establish if he was part of a network.

As many as 400 police were deployed overnight, Hopkins said early Tuesday. He added that residents should expect to see more armed officers on the streets.

If confirmed as a terror incident, it would be the deadliest attack on British soil since the 2005 London bombings , which killed 52 people.

Latest developments

Almost 60 people were also injured in the attack

UK Prime Minister Theresa May: incident an "appalling terrorist attack"

Government to hold emergency Cobra committee Tuesday

Ariana Grande tweets that the incident left her "broken"

Mancunians open their doors, offer rides, to those left stranded by area lockdown

Emergency phone line set up: +44 (0)161 856 9400

How it happened

As concertgoers -- many of them young fans of the singer -- were beginning to leave the arena after Grande's closing set, an explosion rocked the area. Confusion followed as people fled in search of safety, and the sound of wailing sirens soon cut through the smoky air outside the arena.

The incident happened shortly after Grande had left the stage, according to eyewitnesses. Coral Long, the mother of a 10-year-old concertgoer, told CNN's Hala Gorani that they were getting ready to leave the arena when they heard a loud bang from the left side of the arena.

People started running following the explosion, she said. "How we weren't crushed to death is a miracle."

She said her daughter was "devastated -- for her to be 10 years old and witness something like that is just horrific."

Social media posts from the scene showed panicked concertgoers running down the arena's stairs in an attempt to get away.

I never ment to record this but this is me leaving ariana grande #DangerousWomantour #ManchesterArena hope everyone is safe and got out pic.twitter.com/TLu73UBl0x — Rhianna burke (@RhiannaBurke) May 22, 2017

The city's Manchester Royal Infirmary Hospital was blocked off to all but essential staff due to what the facility called a major incident.

Grande, who had just finished the first of three scheduled UK performances when the blast hit, was not injured. She tweeted several hours later: "broken. from the bottom of my heart, i am so so sorry. i don't have words."

broken.

from the bottom of my heart, i am so so sorry. i don't have words. — Ariana Grande (@ArianaGrande) May 23, 2017

After the initial incident, police carried out a precautionary controlled explosion nearby but the item turned out to be discarded clothing, not a "suspicious item," Greater Manchester Police said.

JUST WATCHED Dashcam captures moment of the explosion Replay More Videos ... MUST WATCH Dashcam captures moment of the explosion 00:53

A US official said suicide bombing is now considered to be the "likely" reason for the blast and a male at the scene in Manchester has been identified as the probable bomber.

A US Department of Homeland Security statement said it was "closely monitoring" the situation and added that the department has "no information to indicate a 'specific credible threat involving music venues in the United States.''

Home Secretary Amber Rudd said: "This particular attack was aimed at the most vulnerable in our society, young people, children at a concert." She added that Britons should "remain alert, not alarmed" in the wake of the apparent attack.

The crowd was made up of mainly younger people who had come to see Grande, who is on a European and Latin American tour.

Photos: Deadly explosion at Ariana Grande concert People in Manchester, England, gather in St. Ann's Square on Thursday, May 25. They were observing a national minute of silence to remember the victims of a suicide bombing at an Ariana Grande concert. Hide Caption 1 of 22 Photos: Deadly explosion at Ariana Grande concert Britain's Queen Elizabeth II speaks to 15-year-old Millie Robson and her mother, Marie, during a visit to the Royal Manchester Children's Hospital on May 25. The Queen was visiting those injured in the attack. Hide Caption 2 of 22 Photos: Deadly explosion at Ariana Grande concert A police officer guards a house in Manchester as investigations continued on May 25. Police say a man carrying explosives acted as a lone attacker and died in the blast. Hide Caption 3 of 22 Photos: Deadly explosion at Ariana Grande concert Flowers and tribute messages are left for victims in St. Ann's Square. Hide Caption 4 of 22 Photos: Deadly explosion at Ariana Grande concert A women sheds tears after observing the minute of silence in St. Ann's Square. Hide Caption 5 of 22 Photos: Deadly explosion at Ariana Grande concert A Manchester road is closed off as police raids continued on May 25. Hide Caption 6 of 22 Photos: Deadly explosion at Ariana Grande concert Local residents hold Manchester City and Manchester United soccer jerseys during the national minute of silence. Hide Caption 7 of 22 Photos: Deadly explosion at Ariana Grande concert Police officers deliver flowers to a makeshift memorial in Manchester on Wednesday, May 24. Hide Caption 8 of 22 Photos: Deadly explosion at Ariana Grande concert Women cry after placing flowers in Manchester on May 24. Hide Caption 9 of 22 Photos: Deadly explosion at Ariana Grande concert A forensics team works at the scene of the explosion on Tuesday, May 23. Hide Caption 10 of 22 Photos: Deadly explosion at Ariana Grande concert British Prime Minister Theresa May condemns the "callous terrorist attack" as she delivers a statement in London on May 23. Hide Caption 11 of 22 Photos: Deadly explosion at Ariana Grande concert Police help someone after the attack at Manchester Arena on Monday, May 22. Hide Caption 12 of 22 Photos: Deadly explosion at Ariana Grande concert People gather outside the arena. "We can confirm there was an incident as people were leaving the Ariana Grande show last night," police said on Twitter early on Tuesday. "The incident took place outside the venue in a public space. Our thoughts and prayers go out to the victims." Hide Caption 13 of 22 Photos: Deadly explosion at Ariana Grande concert People receive medical attention at a railway station close to the arena. The incident happened shortly after Grande had left the stage, shortly after 10:30 p.m. (5:30 p.m. ET) according to eyewitnesses. Calvin Welsford, an 18-year-old concertgoer, said that he heard a "loud bang" a couple of minutes after Grande's set had finished. Hide Caption 14 of 22 Photos: Deadly explosion at Ariana Grande concert Paramedics respond to the scene. Hide Caption 15 of 22 Photos: Deadly explosion at Ariana Grande concert Emergency workers and concert attendees gather outside the arena. Hide Caption 16 of 22 Photos: Deadly explosion at Ariana Grande concert People stand by a cordoned-off street close to the arena. Hide Caption 17 of 22 Photos: Deadly explosion at Ariana Grande concert Greater Manchester Police tweeted that emergency services were "responding to (a) serious incident at Manchester Arena. Avoid the area. More details will follow as soon as available." Hide Caption 18 of 22 Photos: Deadly explosion at Ariana Grande concert People are escorted away from the arena. Hide Caption 19 of 22 Photos: Deadly explosion at Ariana Grande concert People sit outside near the arena. The crowd was made up of mainly younger people who had come to see Grande in her first of three scheduled concerts in the UK. Hide Caption 20 of 22 Photos: Deadly explosion at Ariana Grande concert An injured man is helped at the scene. Hide Caption 21 of 22 Photos: Deadly explosion at Ariana Grande concert People hug near armed police who responded to the scene. Hide Caption 22 of 22

Distraught, waiting by the phone

Manchester resident Charlotte Campbell told CNN her 15-year-old daughter, Olivia, went to the concert with a friend, and neither had been in contact since the explosion.

"We've tried everything we can. They're telling us to wait by the phones. Her dad is out looking. ... It's the most horrible feeling ever, to know your daughter is there and you don't know whether she's dead or alive," she said.

Through tears, Campbell said she didn't know how anyone could do this to "innocent children."

"I want her home and I want her safe. ... I just want her to walk through the door."

JUST WATCHED Mom: I don't know if daughter is dead or alive Replay More Videos ... MUST WATCH Mom: I don't know if daughter is dead or alive 01:22

Eyewitness Karen Ford had taken her 13-year-old daughter to the concert.

"It was mayhem on the street, there were children crying, trying to get in touch with parents, parents on their phones trying to contact their kids," she said, adding that cell connection was poor as so many people were using their phones simultaneously.

"I brought my baby home, which some people won't be (able to do) tonight."

A nearby Holiday Inn hotel became a focal point for parents searching for their children who had been at the concert. Some hotels opened their doors to people who could not get home due to an area lockdown.

Taxis and local people offered free rides to those affected.

Manchester Victoria railway station, which is next to the arena, will remain closed Tuesday, UK National Rail announced.

Emergency meeting Tuesday

British Prime Minister Theresa May issued a statement, confirming that police are treating the incident as a terror attack. She has suspended campaigning for June 8 general election and will chair a meeting of the Government's emergency Cobra committee on Tuesday morning.

"We are working to establish the full details of what is being treated by the police as an appalling terrorist attack," the statement read. "All our thoughts are with the victims and the families of those who have been affected."

JUST WATCHED A recent history of UK terror attacks Replay More Videos ... MUST WATCH A recent history of UK terror attacks 01:23

O

ther British political parties followed suit by canceling events planned for Tuesday.

The leader of Britain's Labour Party, Jeremy Corbyn, offered support on Twitter. "Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services."

Stars react

Universal Music Group, parent company for Grande's record label, Republic Records, posted a statement on Facebook.

"We are deeply saddened to learn of tonight's devastating event in Manchester. Our thoughts and prayers are with all those affected by this tragedy," the post says.

Musicians such as Ed Sheeran, Nicki Minaj and Katy Perry all offered supportive tweets.

My heart hurts for my sister, Ariana & every family affected by this tragic event in the U.K. Innocent lives lost. I'm so sorry to hear this — NICKI MINAJ (@NICKIMINAJ) May 23, 2017

CORRECTION: A previous version of this story included a tweet that was incorrectly attributed to singer Nicki Minaj. It has been removed and replaced with the correct tweet.