At least 19 people have died following a serious incident at Manchester Arena, with around 50 others injured.
Emergency services are at the scene following reports of an explosion.
It is being treated as a terrorist incident until police know otherwise.
The venue was tonight hosting a concert by Ariana Grande.
Manchester Police urged people to stay away from the area, saying there were a number of fatalities and injuries.
Latest statement on incident at Manchester Arena pic.twitter.com/BEpLOan3dY
— G M Police (@gmpolice) May 23, 2017
The venue itself said the incident took place in a public space outside the arena as people were leaving the concert.
— Manchester Arena (@ManchesterArena) May 23, 2017
In a statement, British Transport Police said: "Officers are at Manchester Arena following reports of an explosion within the foyer area of the stadium at 10.30pm this evening.
"Emergency services are at the scene and we are working to establish more information regarding the explosion and will provide further updates as soon as possible."
All lines have been closed at Manchester Victoria train station.
Picture by: Peter Byrne/PA Wire/PA Images
Witnesses on social media said they heard 'loud bangs'.
Speaking to Sky News, one witness said: "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.
"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."
In a statement quoted by the New York Times, Ms Grande's publicist said: ”Ariana is O.K. We are further investigating what happened.”
Video shared on social media, meanwhile, showed panicked crowds attempting to leave the arena.
EXPLOSION AT MANCHESTER ARENA AND EVERYONE RAN OUT SO SCARY😠pic.twitter.com/pJbUBoELtE
— ♡♡ (@hannawwh) May 22, 2017
#ManchesterVic - Trains are currently unable to run to / from Manchester Victoria. https://t.co/yftVLlKqrx
— National Rail (@nationalrailenq) May 22, 2017
We are monitoring reports from Manchester. Advice of @gmpolice is to stay away from the area. https://t.co/2473h8qYbs
— Embassy of Ireland (@IrelandEmbGB) May 22, 2017
People stranded by the incident were encouraged to use the #RoomForManchester to find a place to stay.
