Nineteen people have been killed and more than 50 were seriously injured Monday night when an explosion -- possibly caused by a suicide bomber -- hit a Manchester, England concert arena at a show by pop star Ariana Grande, according to law enforcement authorities.

Manchester Police said they are treating the incident as a possible terrorist attack and asked people in the area to stay away.

According to media reports, the explosion occurred about 10:35 p.m. local time in or near a box office/tunnel area at the entrance to the Manchester Arena, where as many as 20,000 people -- many of them young teenagers and children -- were attending the concert.

CNN reported that officials said a male had been identified at the scene as "a possible suicide bomber."

Reuters confirmed Prime Minister Theresa May also said the incident was being treated as a terrorist attack. Reuters noted that if confirmed, it would be the deadliest militant assault on Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005.

Also, it was reported authorities found a suspicious package in a public open space nearby after the explosion at the arena, and were planning to destroy it in a controlled detonation.

Several hours after the bombing, Grande tweeted: "broken. from the bottom of my heart, i am so so sorry. i don't have words."

Manchester police gave a short news briefing about 10 p.m. Eastern, confirming they are investigating the incident as a terror attack; and asking people to stay away from the Manchester Arena.

Earlier in the evening, witnesses reported hearing explosions at the end of the concert, followed by people screaming and running for the exits. Some said they thought the detonation came from outside the arena. Witnesses said the explosion was so powerful, the ground outside the arena shook.

The BBC reported that the North West Counter Terrorism unit was on location and addressing the incident as a possibly terrorist related. Senior counter-terrorism officers in London are also working with authorities.

"We are deeply saddened by this senseless tragedy and our hearts and thoughts are with those impacted by this devastating incident," said Live Nation, the concert's promoter, in a statement, according to Billboard.

Shares of Live Nation Entertainment (LYV) - Get Report , the company that is coordinating Grande's "Dangerous Woman" tour were down almost 4% in after-hours trade.

U.S. futures were slightly lower late Monday evening, but relatively unchanged since the time of the first reports of the incident. Gold was up .06% to $1,262 an ounce, up slightly from the time of first reports.

According to a report by the New York Times, "the BBC interviewed one witness, who was waiting outside the Manchester Arena to pick up his wife and daughter, recounting that the 'whole building shook,' that there was 'carnage everywhere,' and that the explosion appeared to come near the stadium's ticket area. But the BBC emphasized that it was not clear what caused the explosion," the Times reported.

The BBC reported that concertgoers were beginning to leave the arena when when there was a loud explosion and the crowd made a run for the exits, causing a degree of chaos with people pushing past each other.

The Manchester Arena is the largest indoor entertainment facility in Europe and can hold 21,000 people. There were no figures available as to how many attended the Grande show. Republic Records, Grande's label, reported that the 23-year old singer was not injured. The singer is on tour supporting her latest release "Dangerous Woman." Performers Victoria Monet and Bia opened the Manchester show. The tour was due in London Thursday at the O2 Arena.

Though the incident immediately recalled the Paris terrorism attacks in November 2015 at the Eagles of Death Metal concert, there was no official pronouncement of whether terrorism was suspected in the explosion at the Grande concert. But Britain is on "severe" alert, the second highest alert level, and the severe status designates that authorities regard the incident as being caused by militants.

The arena is is next to a train station, Victoria Station. That facility was evacuated according to the Times.