The death toll in the blast at the Manchester Arena in Manchester, UK, rose to at least 22 and the number of wounded to 59, latest reports said.

The blast was carried out by a suicide bomber, who struck as thousands of fans streamed out of a concert by US singer Ariana Grande around 10.30 pm on Monday.

The dead included some children, said reports.

Prime Minister Theresa May said the incident was being treated as a terrorist attack, making it the deadliest militant assault in Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005, said a Reuters report.

Police said the attacker detonated the explosives shortly after 10:33 pm (2133 GMT) at Manchester Arena, which has the capacity to hold 21,000 people.

"We believe, at this stage, the attack last night was conducted by one man," Manchester Chief Constable Ian Hopkins told reporters. "The priority is to establish whether he was acting alone or as part of a network.

"We believe the attacker was carrying an improvised explosive device which he detonated causing this atrocity," said Hopkins, who declined to answer questions about whether the attacker was British.

A video posted on Twitter showed fans, many of them young, screaming and running from the venue. Dozens of parents frantically searched for their children, posting photos and pleading for information on social media.

A witness, attending the concert told news agency Reuters, that she felt a blast as she was leaving the arena. She also said that she heard people screaming and trying to escape.

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

UK Home Secretary Amber Rudd said it was "a barbaric attack, deliberately targeting some of the most vulnerable in our society".

Grande, 23, tweeted: 'Broken. From the bottom of my heart, I am so so sorry. I don't have words."

Paramedics at the scene told the BBC they had treated some of the wounded for "shrapnel-like injuries".

North West Ambulance Service said it had taken 59 casualties from the explosion to hospitals and treated a number of walking wounded.

In light of the attack the Conservatives, Labour, SNP and Liberal Democrats have all confirmed that Tuesday's campaigning in the general election has been suspended.