Armed police gather at Manchester Arena after reports of an explosion at the venue during an Ariana Grande gig in Manchester, England Monday, May 22, 2017.

:: A blast on Monday night at a concert in the English city of Manchester where US singer Ariana Grande had been performing left at least 19 people dead and about 50 injured in what British police said was being treated as a terrorist incident.

Police 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. The first unconfirmed reports of an explosion emerged shortly after 2145 GMT.

British police said that they are treating the blast in Manchester arena as a possible terrorist incident.

A bomb disposal unit arrived outside the Manchester Arena after the reported explosion, British media reported.

A statement from Manchester Arena says that the incident took place outside the venue. The statement reads: “We can confirm there was an incident as people were leaving the Ariana Grande show last night. The incident took place outside the venue in a public space.”

Alex Warren, who was near to where the incident took place told Al Arabiya English the blast “rocked the gate to the Manchester Victoria station entrance as people were leaving” the concert.

“It’s Chaos, and many people are separated from parents” he said, “people are very angry and upset here.”

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.

“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.”

Witnesses reported that many children were at the concert.

A barman at the nearby Steven Charles Snooker Club, who gave his name to the Press Association as Tyler, said he saw people lying on the ground covered in blood.

Meanwhile British train lines out of Manchester Victoria station were blocked, the network rail said.

British Prime Minister Theresa May said on Tuesday that authorities were working to establish the details of the blast, adding that it was being treated as an “appalling terrorist attack”.

“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.”

Buses have been pressed into service to ferry people out of the area; some local taxi companies are also offering free rides for those affected.

Meanwhile, the hastage “Room for Manchester” started trending shortly after the attack as people offer their homes to shelter those who are unable to get home or those who are lost.

A spokesperson for Ariana Grande’s record label said that the singer was “okay”.

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.

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

Medics deploy at the scene of a reported explosion during a concert in Manchester, England on May 23, 2017.