[UPDATE - May 23, 10.40am]
Two US officials said the attack was likely carried out by a suicide bomber, Reuters reported.
British Prime Minister Theresa May said the government is working to establish “the full details of what is being treated by the police as an appalling terrorist attack.”
She said her thoughts are with the victims and the families of those who have been affected. The government is expected to call an emergency Cabinet meeting.
If the incident is confirmed as a terrorist attack it would be the most deadly in Britain since the London subway bombings in 2005.
Nineteen people have been confirmed dead and around 50 injured after an explosion at the end of pop star Ariana Grande's concert in Manchester, northwest England.
Greater Manchester Police tweeted that they were treating this as a terrorist incident until they knew otherwise.
A fleet of ambulances was seen rushing to the Manchester Arena venue after the late Monday incident and police said people should avoid the area.
The deaths followed television footage of people leaving the arena, which had been hosting a concert by the American singer, in a state of alarm and distress.
The first unconfirmed reports of an explosion emerged shortly after 2145 GMT (4.45am HKT).
There had been reports of a loud bang being heard near the end of the concert, causing people to rush for the exits.
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.
Several media outlets reported that there had been two explosions from within the 21,000 capacity venue.
A spokesman for the 23-year-old singer said that she was "okay".
Britain is on its second-highest alert level of "severe" meaning an attack by militants is considered highly likely.