Not less than 22 people have been killed in a suspected suicide bomber attack as thousands of fans streamed out of a concert by U.S. singer Ariana Grande in the English city of Manchester on Monday.

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 At least 59 people were also wounded which included children in the attack.

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.

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. Children were among the dead, police said.

"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 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.

"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 reporters

Ariana Grande, 23, 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. She and Jeremy Corbyn, the leader of the opposition Labour Party, agreed to suspend campaigning ahead of the June 8 election.

Chinese President Xi Jinping sent his condolences to Queen Elizabeth while French President Emmanuel Macron said he would discuss the fight against terrorism with May.