A blast at an Ariana Grande concert in the northern English city of Manchester has killed an unknown number of people and wounded others.

Police said on Monday night they were responding to reports of an explosion and there were a number of confirmed fatalities.

There were no immediate details of what happened during the concert by the American singer.

A video posted on Twitter showed fans screaming and running out of the venue.

“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 news agency.

“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.”

Greater Manchester Police tweeted asking people to stay away.

“Emergency services responding to serious incident at Manchester Arena. Avoid the area. More details will follow as soon as available.”

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 the singer was “okay”.

Britain is on its second-highest alert level of “severe” meaning an attack is considered highly likely.