At least 22 people, including children, are confirmed dead and nearly 60 others injured after an explosion at a concert venue in Manchester, UK on Monday night local time.

Police said the incident, which occurred at Manchester Arena as concertgoers were leaving a show, was a suicide attack carried out by a lone attacker, who died at scene. "We believe, at this stage, the attack last night was conducted by one man," Manchester Chief Constable Ian Hopkins said.

19 people died at the scene and three died in hospital. Some children were confirmed among the dead, police said.

British Prime Minister Theresa May said in a statement that "all thoughts are with the victims and the families of those who have been affected" and officials were seeking details of the blast.

In the aftermath of the explosion, May's Conservatives are suspending campaigning for a June 8 national election. An emergency security meeting will be held at Downing Street at 0900 BST on Tuesday, Reuters reported.

Greater Manchester Police said in a statement that the blast is being treated as a "terrorist incident" and they are working closely with the national counter terrorism police network and UK intelligence partners. 

Screenshot of a tweet by Greater Manchester Police

Police also carried out a "controlled explosion" in the Cathedral Gardens area near the venue, after a suspected explosive device was spotted. But police later said that the item was abandoned clothing.

Screenshot of a tweet by Greater Manchester Police

Reuters reports that Britain is on its second-highest alert level of "severe" meaning an attack by militants is considered highly likely.

The explosion took place shortly after 2230 BST as US pop singer Ariana Grande was holding a concert. "The incident took place outside the venue in a public space," Manchester Arena said on its official account on Twitter.

Armed police at Manchester Arena after reports of the explosion at the venue on May 22, 2017. /VCG Photo

Ambulances were seen rushing to the area and a significant number of casualties were treated by paramedics.

Police warned in a statement that people should avoid the area.

London Mayor Sadiq Khan on Tuesday condemned the attack in a statement published on his Twitter account, calling it a "cowardly act of terrorism." He said police are reviewing security arrangements in London and more police officers will be deployed on streets.

Statement by London Mayor Sadiq Khan