LONDON - Police say 19 people are confirmed dead and around 50 others are injured following reports of an explosion Monday night at an Ariana Grande concert in Manchester, England. 

The incident, at Manchester Arena, is being treated as a terrorist incident until police know otherwise, Greater Manchester Police said in a statement. 

A representative said Ariana Grande was not injured.

A U.S. law enforcement source who received a preliminary briefing said the situation is fluid but that there were two explosions that contained nails, CBS News investigative producer Pat Milton reports. The source said there are more than 20 casualties, apparently including a large number of young girls. 

The source said that one person was detained in a vehicle but it is not clear what, if any, involvement that potential suspect may have had in the incident.

Authorities are investigating whether the attack was a possible suicide bomber, a U.S. law enforcement source says, but there has been no confirmation. The source stressed the investigation is in the initial stages and facts have not been confirmed.

Joseph Carozza, a representative from Grande's U.S. record label, said the singer is OK and they are investigating what happened.

Britain's terrorist threat level has been set at "severe" in recent years indicating an attack is highly likely. 

Witnesses reported hearing two loud bangs coming from near the arena's bars at about 10:35 p.m. but there were few other details. Emergency vehicles were helping the injured and bomb disposal units were later seen outside the venue.

"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."

Oliver Jones, 17, said, "The bang echoed around the foyer of the arena and people started to run."

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 canceled.

"Due to an incident involving the emergency services [Manchester Victoria] has been evacuated," Northern train service officials said. "We will provide further updates when possible."

Zach Bruce, who was at the concert, described the scene on the phone to CBSN. 

"It was one loud bang -- it was so loud," Bruce said. 

Asked if he would describe it as an explosion, he said yes -- "It was an explosion." 

He described people panicking. "I didn't know what was going on," he said.

One Twitter user said she heard an "explosion" from inside the area, where Grande performed moments earlier. There were no immediate details of what happened during the concert.

Video from inside the arena showed concertgoers screaming as they made their way out amid a sea of pink balloons.

One concert goer told BBC Radio 5 Live she was one of first people to exit the stadium. She said she heard an explosion toward the end of the performance. There were an estimated 20,000 people inside the arena on Monday night.

The U.S. Embassy in London issued a statement Monday alerting U.S. citizens to "maintain security awareness" and seek guidance from local authorities.?

"We strongly encourage U.S. citizens in the United Kingdom to directly contact concerned family members in the United States to advise them of ?your safety," the statement read.

U.K. Prime Minister Theresa May said in a statement that officials "are working to establish the full details of what is being treated by the police as an appalling terrorist attack."

"All our thoughts are with the victims and the families of those who have been affected," May said.

Jeremy Corbyn, leader of the Labour Party, responded to reports of the incident on Twitter. 

"Terrible incident in Manchester," he said. "My thoughts are with all those affected and our brilliant emergency services."

The Dangerous Woman Tour is the third concert tour by Grande and supports her third studio album, "Dangerous Woman."

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.