At least 19 dead following 'terrorist incident' at Ariana Grande concert in Manchester

"This is currently being treated as a terrorist incident until police know otherwise," the Greater Manchester Police said in a statement.

Witnesses interviewed by the BBC reported hearing a loud blast following a performance by American pop singer Ariana Grande at Manchester Arena. Police said the blast occurred around 10:30 p.m.

Initial evidence at the scene suggested the attack may have been a suicide bombing, according to two U.S. security officials, who spoke on the condition of anonymity to discuss an ongoing investigation. British authorities, who were meeting in emergency sessions across Manchester and London, did not immediately confirm those reports.

If confirmed as a terrorist attack, it would be the worst strike on British soil since 2005, when bombers killed 54 people on London trains and buses.

Cellphone video showed chaotic scenes of people screaming and running in the aftermath of the blast. The arena was packed with attendees and pink balloons that had fallen from the ceiling during the concert's final song. Initially, concertgoers said they thought popping balloons had set off a panic.

But witnesses later reported seeing the prone bodies of those who had been wounded and killed, as well as others who were streaked with blood and were staggering away from the scene. Some were injured in the rush to get out.

The hospital, Wythenshawe, said it was dealing with "mass casualties."

Heavily armed police and emergency services swarmed the arena, with ambulances - their blue lights flashing - rushing to the scene.

The local emergency-response service advised the public to call only "for life-threatening emergencies."

Many of those attending the concert were teenagers. Witnesses reported that outside the arena, parents were frantically attempting to locate their children.

A father told the BBC that he was leaving the arena with his wife and daughter when the blast blew him through a set of doors. Afterward, the man, identified as Andy, said he saw about 30 people "scattered everywhere. Some of them looked dead."

Separated from his wife and daughter, he said, he "looked at some of the bodies trying to find my family."

He later found them, uninjured.

"It was really scary," Michelle Sullivan, who was attending the concert with her 12- and 15-year-old daughters, told the BBC. "Just as the lights have gone down, we heard a really loud explosion. ... Everybody screamed."

"When we got out, they just said, 'Keep on running, keep on running.' "

Karen Ford, a witness, told the BBC that "there were kids outside, crying on the phone, trying to find their parents."

Concertgoers said that they saw nuts and bolts littering the ground near the blast scene and that the smell of explosives hung heavily in the air.

The singer was "OK," a spokesman for Grande's record label told the Reuters news agency.

Around 1:30 a.m., police announced that there would be a controlled explosion after a suspicious device was found. A loud bang was heard minutes later.

The arena has a capacity of 21,000. Manchester transport police said the explosion occurred in the arena's foyer, where people were congregating to buy concert merchandise. Manchester Arena said the attack took place just outside the facility, in a public space.

Though nobody immediately claimed responsibility for Monday's violence, scenes of bloodied, panicked concert-goers streaking for safety brought to mind similar images at the Bataclan theater in Paris in November 2015. The concert hall became the scene of extreme carnage after multiple gunmen burst in during a show by the American rock band Eagles of Death Metal and began shooting. The attack - for which Islamic State later claimed responsibility - killed 89 people and injured hundreds more, becoming the deadliest event on French soil since World War II.

Britain has had fewer terrorist attacks in recent years than many of its European neighbors. Monday night's blast came two months after a careening driver left four people dead on London's Westminster Bridge, then stabbed to death a police officer at the gates of Parliament.

Grande is a 23-year-old pop singer and actress who has been in the public spotlight since 2010, when she began appearing on the Nickelodeon television show Victorious. More recently, the former teen idol has been touring to promote her third studio album, "Dangerous Woman." She has sold more than 1.7 million albums in recent years.