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".
Greater Manchester Police tweeted: "Police responded to reports of an incident at Manchester Arena. Please stay away from the area."
The area around the arena was swamped with police and emergency services. Approach roads have been closed by police.
British Transport Police said that emergency services were still working to "establish more information regarding the explosion and will provide further updates as soon as possible".
Manchester's Victoria station, which backs onto the arena, has been evacuated and all trains cancelled.
National Rail said in an online statement: "Emergency services are dealing with an incident at Manchester Arena. As Manchester Victoria is located near the arena, the station has been evacuated and all lines closed.
"Trains are currently unable to run to / from Manchester Victoria. Some trains will be cancelled throughout or start / terminate at alternative stations. Disruption is expected to continue until end of the day."
While Twitter users have posted hearing a 'gun shot' or 'bomb', initial sources had reported that it might be a speaker that blew during the performance or even balloons that had popped.
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.
Another man who took his daughter to the gig said: "Ariana said: 'Bye bye Manchester!' The lights came on then there was two big noises and people started running and shouting.
"We got in a lift to go up to the next floor to the exits. When the doors opened I saw blood on the floor and two injured people. Blood all over their faces. A lot of people were in shock."
Armed police have now swamped the streets around the arena and police are moving members of the public away from the area.
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."
A barman at the nearby Steven Charles Snooker Club, who gave his name as Tyler, said he saw people lying on the ground covered in blood.
"We've had a few people in with panic attacks and in all kinds of disarray," he told Press Association.
"We've got four girls here - trying to get them sorted to get picked up.
"There was a gentleman on the floor with his leg all bleeding and woman with blood down one side of her face.
"We felt something but didn't know what it was - there was a sound like thunder.
"One girl had a panic attack and another had streaming tears, a woman had a heart attack just outside.
"It's a lot of teenagers - they're all in tears."
Suzy Mitchell (26), whose flat is opposite the venue, reported a huge bang rocking the neighbourhood.
She told the Press Association: "(I) just heard a huge bang from my bed, came out to the front of my apartments (we're on the top floor so have perfect view) and everyone was running away in big crowds.
"The bang was so big I heard it from my room which is at the back of the apartment blocks.
"Currently lots of emergency services going to and from. But can't see anything substantial as of yet except fleeing people and lots of cars."
