At least 22 people, including children, were killed and more than 50 wounded in an explosion at the end of a concert by US singer Ariana Grande in the English city of Manchester on Monday, in what is being seen as a suicide bombing.

Prime Minister Theresa May said the incident was being treated as a terrorist attack. If confirmed, it would be the deadliest militant assault in Britain since four bombers killed 52 people in suicide bombings on London's transport system in July 2005.

Police responded to reports of an explosion shortly after 10:35 pm (2135 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.

The man behind the attack in Manchester died when he detonated his device, police said on Tuesday.

Greater Manchester Police chief Ian Hopkins said the man had set off an “improvised explosive device” as the audience was leaving the concert.

"We believe, at this stage, the attack last night was conducted by one man," Hopkins said.

"The priority is to establish whether he was acting alone or as part of a network."

"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," he added.

Hopkins said police are treating the blast as an act of terrorism “until we know otherwise.”

He added that investigators were "working closely with the national counter-terrorism policing network and UK intelligence partners".

Suspected bomber identified: US officials

The suspected suicide bomber behind the attack has been identified as Salman Abedi, US officials told Reuters on Tuesday.

Two of the officials who have been in contact with British authorities said the suspect has been identified as Salman Abedi or Salman Ramadan Abedi and was believed to have travelled to Manchester from London by train.

A third US government source said the bomber had been identified as Salman Abedi.

Witness reports

A witness who attended the concert said she felt a huge blast as she was leaving the arena, followed by screaming and a rush by thousands of people trying to escape the building.

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.

"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."

A spokesman for Ariana Grande, 23, said the singer was "okay". The singer later said on Twitter: "broken. from the bottom of my heart, i am so so sorry. i don't have words."

May, who faces an election in two-and-a-half weeks, said her thoughts were with the victims and their families. Her ruling Conservative Party was preparing to suspend campaigning ahead of the election due to the suspected attack.

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

Election campaigns suspended

British Prime Minister Theresa May and main opposition leader Jeremy Corbyn have said they are suspending their election campaigns “until further notice” on Tuesday after a suspected terror attack.

Labour leader Corbyn said he had spoken to Conservative leader May and they had agreed that all national campaigning ahead of the June 8 vote would be suspended, the Press Association news agency reported.

Trump condemns 'evil losers' behind attack

US President Donald Trump condemned the “evil losers” behind the attack and called on leaders in the Middle East in particular to help root out violence

“So many young, beautiful, innocent people living and enjoying their lives murdered by evil losers,” Trump said after meeting Palestinian president Mahmud Abbas in the occupied West Bank.

“I won't call them monsters because they would like that term. They would think that's a great name.”

Trump added that “I will call them from now on losers because that's what they are. They're losers, and we'll have more of them. But they're losers ─ just remember that.”