A number of people have died and others were injured in a reported explosion at Manchester Arena during a pop concert, police say.
Witnesses reported hearing a "huge bang" at the venue during an Ariana Grande gig on Monday evening.
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. 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.
One 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."
Jade Baynes, 18, from Hull, was told to run from the area by armed police after leaving the concert arena.
She said she heard loud bangs and what sounded like gunshots just after the entertainment had finished.
She added: "There were just a loud bang and a flash and everyone tried to scramble out.
"An alarm came on telling everyone to stay calm but leave as quickly as possible."
Jade and her friend Jasmine Mia, 21, also from Hull, said there appeared to be some sort of commotion in the first tier of the arena, with a number of stewards stood around the seats.
Armed police have now swamped the streets around the arena and police are moving members of the public away from the area.