At least 19 people were killed and more than 50 injured in an explosion at the end of a concert by US singer Ariana Grande in the English city of Manchester on Monday.
Two US officials said a suicide bomber was suspected.
Prime Minister Theresa May said the incident was being treated as a terrorist attack. If confirmed, it would be the deadliest militant assault on Britain since four British Muslims 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 a capacity for 21,000 people, and where the U.S. singer had been performing to an audience that included many children.
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," concertgoer 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".
May, who faces an election in two and a half weeks, said her thoughts were with the victims and their families.
"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."