"Horrendous:" Musicians, celebrities, politicians weigh in on deadly incident in Manchester

MANCHESTER, England -- An explosion struck an Ariana Grande concert in northern England late Monday, killing at least 19 people and injuring dozens in what police say they are treating as a terrorist attack.

Greater Manchester Police said 19 people were confirmed dead and roughly 50 were injured by the explosion at Manchester Arena. Emergency vehicles were helping the injured and bomb disposal units were later seen outside the venue.

There was mass panic after the explosion at the end of the concert, which was part of Grande's The Dangerous Woman Tour. The singer was not injured, according to a representative.

Britain's terrorist threat level has been set at "severe" in recent years indicating an attack is highly likely. Police said the explosion is being judged a terrorist attack unless new information proves otherwise.

Witnesses reported hearing two loud bangs coming from near the arena's bars at about 10:35 p.m. but there were few further details.

Celebrities and politicians are weighing in on social media after this deadly incident: