A blast on Monday night at a concert in the northern English city of Manchester where U.S. singer Ariana Grande had been performing left an unknown number of people dead and injured, police said.

Police said they were responding to reports of an explosion and that there were a number of confirmed casualties and others injured.

"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."

Manchester Arena, the largest indoor arena in Europe, opened in 1995 and has a capacity for 21,000 people, according to its website. It is a popular concert and sporting venue.

A spokesman for Ariana Grande's record label said that the singer was "okay". A video posted on Twitter showed fans screaming and running out of the venue.

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

Evie Brewster, who attended the concert, told MailOnline: 'Ariana Grande had just finished her last song and left the stage when a huge explosion sounded.

'Suddenly everybody started screaming and running for the exit.'

Katy Perry was among the pop stars tweeting their prayers. 

(Reporting by Alistair Smout; Writing by Guy Faulconbridge; Editing by Sandra Maler)