At least 20 people were killed Monday, and hundreds injured, in an explosion at Manchester Arena at the end of an Ariana Grande concert, according to preliminary findings.

Video footage showed thousands of people fleeing in tears from the venue, many covered in blood, after the Ariana Grande concert finished.

Armed officers surrounded the venue and bloodied concertgoers were pictured being helped by emergency services outside the gig.

On social media, people shared videos of people rushing from the arena and ambulances arriving.

David Richardson, who attended the show with his 13-year-old daughter, told the MEN that he heard two explosions just after the show.

“It was about 40 feet behind us near one of the exits. We just thought it was people messing about then it happened again. Another explosion sounded. Then we saw the smoke. Everyone just fled.”

Majid Khan, 22, said she heard a “huge bomb-like bang that hugely panicked everyone and we were all trying to flee the arena.”

“Everyone was in a huge state of panic, calling each other as some had gone to the toilet whilst this had gone off, so it was just extremely disturbing for everyone there.”

Another witness, Oliver Jones, 17, told Sky News he was in the bathroom when he heard “a loud bang.”

“The bang echoed around the foyer of the arena and people started to run,” Jones said. “I (saw) people running and screaming towards one direction and then many were turning around to run back the other way. Security was running out as well as the fans and concert goers.”

Manchester Victoria station, which is under the arena, was also evacuated, though there were no initial reports of injuries there.

Ambulances out in force heading to Manchester Arena after all lining up on Thompson street fire station. pic.twitter.com/vpAFFUG0MI

— Sam Ward (@SamWardMCR) May 22, 2017

BREAKING |

Line of ambulances heading to #manchester arena where a major #explosion has happenedpic.twitter.com/AGfAw6H7Wo

— Vocal Europe (@thevocaleurope) May 22, 2017

(AP)