UK police on Tuesday raised the death toll in a blast at an Ariana Grande concert attended by thousands of young fans in Manchester, northern England, to at least 22 people, reiterating that they were treating it a "terrorist incident".

Ian Hopkins,chief constable for Great Manchester, said children were among those confirmed dead in Monday night's explosion at Manchester Arena, which also left at least 59 injured.

Police believe the explosion was caused by one man "carrying an improvised explosive device, which he detonated causing this atrocity", Hopkins told reporters on Tuesday morning.

If confirmed, it would be the deadliest attack on Britain since four men killed 52 people in suicide bombings on London's transport system in July 2005.

Police cars, bomb-disposal units and 60 ambulances raced to the scene at Manchester Arena after Monday night's explosion, which struck outside the venue as concert-goers were leaving.

The incident comes ahead of a June 8 general election. Early on Tuesday, UK politicians said they were suspending election campaigning until further notice following the events in Manchester.

Police responded to reports of an explosion shortly after 10:35pm (21:35 GMT) at the arena, which has the capacity to hold 21,000 people, where the US singer had been performing to an audience that included many children.

"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena," concert-goer Majid Khan, 22, told Britain's Press Association.

Catherine Macfarlane told the Reuters news agency the blast hit after the concert was over.

"We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming," Macfarlane said.

"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."

Greater Manchester Police tweeted asking people to stay away.

"The incident took place outside the venue in a public space," police said. "Our thoughts and prayers go out to the victims."

The local ambulance service said on Twitter it had taken 59 casualties from the incident and treated "a number of walking wounded on scene".

A "precautionary controlled explosion" was carried out near the venue on a package that was believed to be a suspect device, but turned out to be "abandoned clothing, not a suspicious item".

Manchester Arena, which opened in 1995, is the largest indoor arena in Europe, according to its website.

A spokesman for Ariana Grande's record label said the singer was "okay". The concert was part of Grande's The Dangerous Woman Tour.

Grande tweeted that she was "broken" after the incident.

Paula Robinson, 48, was at the train station next to the arena with her husband when she felt the explosion and saw dozens of teenage girls screaming and running away from the arena.

Robinson took dozens of teenage girls to the nearby Holiday Inn Express hotel and tweeted out her phone number to worried parents telling them to meet her there. She said her phone has not stopped ringing since her tweet.

"Parents were frantic running about trying to get to their children," she said.

British Prime Minister Theresa May condemned the "appalling terrorist attack".

"We are working to establish the full details of what is being treated by the police as an appalling terrorist attack," May said in a statement.

May, who cancelled campaign events on Tuesday, will hold a meeting of the country's top security committee at 9am (08:00 GMT).

Labour leader Jeremy Corbyn commented on Twitter, saying: "Terrible incident in Manchester. My thoughts are with all those affected and our brilliant emergency services."

Corbyn also said that his party had also put campaign events on hold. The leaders of the Liberal Democrats, Scottish National Party and UK Independence Party also suspended their election campaigning.