UPDATE - May 22, 2017: MSNBC reports U.K. authorities' forensic evidence points to possible suicide bomber.
LONDON — An explosion struck an Ariana Grande concert in northern England late Monday, killing at least 19 people and injuring dozens in what police say they are treating as a terrorist attack.
Greater Manchester Police said 19 people were confirmed dead and roughly 50 were injured by the explosion at Manchester Arena. Emergency vehicles were helping the injured and bomb disposal units were later seen outside the venue.
Police and fans close to the Manchester Arena on May 23 in Manchester. (Photo: Dave Thompson/Getty Images)
There was mass panic after the explosion at the end of the concert, which was part of Grande's The Dangerous Woman Tour. The singer was not injured, according to a representative.
Britain's terrorist threat level has been set at "severe" in recent years indicating an attack is highly likely. Police said the explosion is being judged a terrorist attack unless new information proves otherwise.
Witnesses reported hearing two loud bangs coming from near the arena's bars at about 10:35 p.m. but there were few further details.
"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena," concertgoer Majid Khan, 22, told Britain's Press Association. "It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit."
Added Oliver Jones, 17: "The bang echoed around the foyer of the arena and people started to run."
Video from inside the arena showed concertgoers screaming as they made their way out amid a sea of pink balloons.
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.
May also cancelled campaign events Tuesday after the blast, which killed at least 19 people and injured more than 50. She is due to chair a meeting of the government's emergency committee, COBRA, later.
Liberal Democrat leader Tim Farron cancelled a campaign tour to Gibraltar after the explosion, which police say they are treating as a terrorist attack.
Britons are due to go to the polls on June 8.
If the incident is confirmed as a terrorist attack it would be the most deadly in Britain since the London subway bombings in 2005.
Police advised the public to avoid the area around the Manchester Arena, and the train station near the arena, Victoria Station, was evacuated and all trains cancelled.
The U.S. Department of Homeland Security says there is no evidence of credible threats against music venues in the U.S. The department says the public may experience increased security in and around public places and events.
DHS says it is closely monitoring the situation at Manchester Arena and working with U.K. officials to obtain additional information about the cause of the explosion.
Joseph Carozza, a representative from Grande's US record label, said the singer is OK and they are investigating what happened.
Grande sent a tweet a few hours after the incident, saying she is "broken" and "so so sorry."
The Dangerous Woman Tour is the third concert tour by Grande and supports her third studio album, "Dangerous Woman."
Police stand by a cordoned off street close to the Manchester Arena on May 22 in Manchester.
Grande's role as Cat Valentine on Nickelodeon's high school sitcom "Victorious" propelled her to teen idol status, starting in 2010.
The 23-year-old Grande, with her signature high ponytail, went on to also star in spinoffs that included "iCarly," as she worked to develop her recording career.
The tour began in Phoenix in February. After Manchester, Grande was to perform at venues in Europe, including Belgium, Poland, Germany, Switzerland and France, with concerts in Latin America and Asia to follow.
AP Music Writer Mesfin Fekadu contributed to this story from Jersey City, New Jersey.
With files from Rebecca Zamon
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