Kim Hjelmgaard, and Melanie Eversley

USA TODAY

LONDON — A deadly explosion at an Ariana Grande concert is believed to be the work of an apparent suicide bomber, police said early Tuesday morning, killing at least 22 people and sending thousands of terrified concert-goers running for the exits as chaos unfolded in a Manchester arena.

At least 50 others were injured and photos from the scene showed carnage as scores of ambulances and paramedics rushed to the area. Grande, who was just finishing her performance when the explosions happened around 10:35 p.m. local time, was not among the injured.

The pop singer issued a tweet late Monday night expressing sorrow for the events. She tweeted: "broken. from the bottom of my heart, i am so so sorry. i don't have words."

The Greater Manchester Police Department reported via Twitter that the agency is treating the incident as a terror attack until proven otherwise. British authorities are focusing on the attack as a possible suicide bombing, according to a U.S. law enforcement official briefed on the matter.

“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, which were dropped from the rafters as Grande's signature finale just moments before the explosion.

Witness Kiera Dawber tearfully told CNN that there was a "massive, massive explosion" followed by a chaotic scene of screams, shouts, shoes and handbags strewn about, and about 20 bodies on the floor of a hallway. "You could see straight off that they were just dead," Dawber said.

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Outside, traffic was at a standstill as fleeing concert goers ran through the streets, Dawber said.

Prime Minister Theresa May said her thoughts were with the victims and their families in “what is being treated by the police as an appalling terrorist attack.”

The horrific images from the scene have already drawn comparison to the November 2015 terror attacks in Paris, which in particular targeted a concert hall. Those attacks killed 130 people, including dozens trapped inside an Eagles of Death Metal concert in the Bataclan theatre,

The Department of Homeland Security said after the explosion that U.S. citizens should "maintain security awareness."

"At this time, we have no information to indicate a specific credible threat involving music venues in the United States," DHS said in a statement. "However, the public may experience increased security in and around public places and events as officials take additional precautions."

Police departments in the U.S., including Chicago, are monitoring the Manchester situation.

Chicago police are "closely monitoring the incident in Manchester and we are in real-time communication with our federal partners," the department said. "At this time, there is no nexus or known threat to the Chicago area but as always, we encourage individuals to report any suspicious activity to 911. Our thoughts and prayers go out to all of those affected by this horrible tragedy."

New York Gov. Andrew Cuomo said he was "heartbroken" by the explosion in Manchester and said that state law enforcement would step up security and patrols.

Eyewitnesses told local television that the Grande concert was just wrapping up when at least one thunderous explosion could be heard. Manchester Arena holds 21,000 people.

So far no one has claimed responsibility for the apparent attack. But Rita Katz, a terrorism analyst and the co-founder of the Search International Terrorist Entities (SITE) Intelligence Group, a private intelligence firm in Washington, D.C., said that the terror group ISIS has celebrated the attack on social media.

The tragedy brought out best in some local residents, who took to the Facebook page of the Manchester police to offer free rides and even their homes to panicked concert goers.

The attack appears to have been either in the main hallway of the arena, or just outside the building, British Transport Police said. Emergency responders set up a triage area in the arena.

Twitter was abuzz with video of panicked fans running out of the Manchester Arena. A Grande label rep told Variety that there were two loud bangs at the concert venue.

Follow Melanie Eversley on Twitter at @melanieeversley. Follow Kim Hjelmgaard on Twitter at @khjelmgaard.

Contributing: Kevin Johnson