Police have said that at least 19 people have been killed more than 50 injured after what is being investigated as a possible terrorist attack at the Manchester Arena.

As the authorities scrambled to deal with what, if confirmed be the deadliest terror attack in the UK since the July 7 bombing in 2005, officials said they were for now considering the incident that took place shortly after an Ariana Grande concert to be a terror incident.

“So far 19 people have been confirmed dead, with around 50 others injured. This is currently being treated as a terrorist incident until police know otherwise,” Greater Manchester Police said in a tweet.

Police had earlier said they were responding to reports of an explosion and that there were a number of confirmed fatalities and others injured at the arena, which has a capacity for 21,000 people.

A witness who attended the concert said she felt a huge blast as she was leaving the arena, followed by screaming and a rush as thousands of people trying to escape.

(Getty Images)

“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.”

Dashcam video appears to show moment of Manchester explosion

Witnesses reported that many children were at the concert.

Manchester Arena, the largest indoor arena in Europe, opened in 1995 and is a popular concert and sporting venue.

A spokesman for Ariana Grande, 23, said the singer was “okay”. A video posted on Twitter showed fans, many of them young, screaming and running from the venue.

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