• 19 dead in explosion at Ariana Grande concert in Manchester, England.

• Police treating event as suspected terrorist incident, with reports saying it could have been a suicide bomber.

• Prime Minister Theresa May said officials are "working to establish the full details" of the incident.

• Ariana Grande tweeted, "Broken. From the bottom of my heart, I am so so sorry. I don't have words."

(Armed police outside Manchester Arena.PA)

At least 19 people have died and 59 others were injured in an explosion at Manchester Arena on Monday evening in what police are treating as a terrorist attack.

Police have not confirmed the cause of the explosion, which occurred around 10:30 p.m. local time (BST) at an Ariana Grande concert.

Greater Manchester Police Chief Constable Ian Hopkins said the force is working closely with counter-terrorism and intelligence officials as it investigates. He said officers are "still receiving information," and a further update will be provided when they have "a clearer picture."

British Prime Minister Theresa May said officials are "working to establish the full details" of the what police believe is an "appalling" terrorist attack.

"All our thoughts are with the victims and the families of those who have been affected," she said in the immediate aftermath of what appeared to be the deadliest incident of terror in Britain since the 2005 London subway bombings.

(Police and fans close to the Manchester Arena.Dave Thompson/Getty Images)

Manchester police warned residents to avoid the area surrounding the arena, where Grande had just finished a concert. The American singer is safe, according to her spokesperson.

"Broken. From the bottom of my heart, I am so so sorry. I don't have words," she tweeted later.

Grande's manager, Scooter Braun, followed up with a statement, saying in part: "We mourn the lives of children and loved ones taken by this cowardly act. We are thankful for the selfless service tonight of Manchester's first responders who rushed towards danger to help save lives."

Police detonated a second device in a controlled explosion shortly after the initial event. It was "abandoned clothing" and not a suspicious item.

There are a number of reports, including from Reuters, that the first explosion could have been caused by a suicide bomber, although that was not immediately confirmed by local police. Eyewitnesses have suggested that the bomb could have contained nails and other metal items.

The initial explosion caused concertgoers to scramble out of the concert building, and videos posted on social media showed police cars and ambulances rushing to the scene. British transport police said the explosion occurred in the "foyer area," or lobby, of the Manchester Arena, which has a capacity of 21,000.

(Two women wrapped in thermal blankets stand near the Manchester Arena.REUTERS/Andrew Yates)

Kim Jackson, a witness at the scene, described the incident to Business Insider.

"We had floor seats and were just heading to exit. There was a massive bang from towards the back of the arena from the corridor area," she said.

"Everyone went silent and then huge crowds of people came running from the back and everyone was screaming and crying. We just wanted to get out of there as quickly as possible so we just headed straight for the exit. When we got up to the corridor there was smoke and we could smell burning. We just ran outside and got as far away as we could."

Speaking to Manchester Evening News, concertgoer Sibion Joyce said: "The concert had ended and she had played her last encore. She went off and the lights came on. Everyone got up to leave. As we did we just heard a loud bang. People were running and people were really scared."

(Emergency services arrive close to the Manchester Arena.Dave Thompson/Getty Images)

(Armed police officers stand next to a police cordon outside the Manchester Arena.REUTERS/Andrew Yates)

Trains were canceled to and from Manchester Victoria train station, according to National Rail, while the station approach was reportedly closed by police.

Concertgoers were offered accommodation by locals, with people using the #roomformanchester on Twitter to help those in the city stranded without a place to stay. Meanwhile, a desperate hunt for missing people is underway.

Manchester Mayor Andy Burnham said on Twitter: "My heart goes out to families who have lost loved ones, my admiration to our brave emergency services. A terrible night for our great city."

Manchester Central MP Lucy Powell added:

The incident comes in the thick of a UK general election campaign, which will be put on hold on Tuesday out of respect for the victims in Manchester.

Labour leader Jeremy Corbyn was one of the first to comment on events, saying his "thoughts are with all those affected." Liberal Democrat leader Tim Farron added: "Deepest condolences to the victims and families in Manchester."

Video captures the moment of the explosion:

Another showed how the blast caused pandemonium inside the Manchester Arena and people ran for safety:

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