Police have confirmed up to 19 people have died following a reported explosion at Manchester Arena.
They said at least 50 other people were injured after the major incident at the end of the Ariana Grande gig.
GMP said at 1am that they are currently treating the incident as terror related.
They said: "Just before 10:35pm police were called to reports of an explosion at Manchester Arena.
"So far 19 people have been confirmed dead with around 50 others injured.
"This is currently being treated as a terrorist incident until police know otherwise."
Witnesses took to social media to say they heard a “huge bang” followed by another blast at the venue on Monday evening.
It has since been reported that the explosion could have been a nail bomb.
Greater Manchester Police said there are already a “number of confirmed fatalities and others injured” and emergency services are at the scene after “reports of an explosion”.
The area around the arena was swamped with police and emergency services.
Armed police were spotted on the street and the approach roads have been closed by police. Witnesses said they heard two loud bangs inside the arena and coming from where the bars are located.
Manchester’s Victoria station, which backs onto the arena, has been evacuated and all trains cancelled.
Michelle Sullivan, from Huddersfield, told the BBC she was attending the concert with her daughters, aged 12 and 15.
"It was really scary," she said. "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'."
Another fan at the concert described how the explosion hit the venue as fans were leaving the building.
Majid Khan, 22, said: “I and my sister, along with a lot of others were seeing Ariana Grande perform at Manchester Arena, and we were all exiting the venue when around 10.40-10.45pm-ish a huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena.
“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 Trinity Way and that was blocked so everyone was just running to any exit they could find as quickly as they could.
“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.”
David Richardson was at the gig with his daughter Emily, 13. He said: “The lights came on after the gig and people were starting to leave. As I turned to the left there was an explosion. It was about 40ft 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. Some people were injured. We saw blood on people when we got outside. People were just running all over the place.
“People have said it was a speaker or balloons but it wasn’t. It was explosions.”
The situation is ongoing.
Ariana Grande’s spokesperson has confirmed that the singer was unhurt in the explosion.
GMP tweeted a statement which said: “Emergency services are currently responding to reports of an explosion at Manchester Arena.
“There are a number of confirmed fatalies and others injured.
“Please AVOID the area as first responders work tirelessly at the scene.
“Details of a casualties bureau will follow as soon as possible.”