Nineteen people have been killed in a "serious incident" at an Ariana Grande concert in Manchester, police have said. More than 50 others were injured.
Hundreds of people were seen fleeing Manchester Arena following reports of two large bangs on Monday evening. Video posted on Twitter showed fans streaming out of the venue.
The police bomb squad later carried out a controlled explosion in a nearby park following the incident, but said the suspicious package turned out to be abandoned clothing.
The BBC has reported that senior counterterrorism officers were assembled and were seen liaising with the Home Office in London.
Police have urged people to avoid the area and said the blast was being treated as a terrorist incident.
Unconfirmed footage from a car dashboard camera posted on Twitter showed a flash of light following a loud bang at the arena.
Train lines out of nearby Victoria Station were blocked following the incident, authorities said.
'Huge state of panic'
Concert attendee Majid Khan, 22, told The Guardian newspaper a bomb-like bang went off as people were exiting the 18,000-capacity venue, leading to a mass panic.
"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," he was quoted as saying.
Eyewitness Sasina Akhtar told Manchester Evening News there was an explosion at the back of the arena as Grande performed her final song.
"We saw young girls with blood on them, everyone was screaming and people were running. There was lots of smoke."
"We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming," concertgoer Catherine Macfarlane told Reuters.
"It was a huge explosion - you could feel it in your chest. It was chaotic. Everybody was running and screaming and just trying to get out."
PM suspends election campaign
About four hours after the explosion, Prime Minister Theresa May released a statement condemning the "terrorist attack."
"We are working to establish the full details of what is being treated by the police as an appalling terrorist attack. All our thoughts are with the victims and the families of those who have been affected."
May's Conservative party has said it will suspend campaigning for the upcoming national election on June 8.
Hospitals expect mass casualties
Ambulances streamed to the scene as the regional NHS Trust said they were responding to the incident. The BBC has reported that people were being treated for "shrapnel-like injuries."
The Manchester Evening News reported that staff at Wythenshawe Hospital in South Manchester told people without life-threatening injuries to leave the emergency room.
"Anybody who is not serious, can you please leave and get treatment elsewhere as there have been reports of an explosion in Manchester, weâ€™re expecting mass casualties," a nurse reportedly said.
A spokesman for Ariana Grande said the singer was "OK" and that they were investigating what had happened.
The US singer has wide appeal among a young audience, after finding success on the children's TV channel Nickelodeon.
Early Tuesday morning, Grande spoke of her sorrow on Twitter.
broken.â€” Ariana Grande (@ArianaGrande) May 23, 2017
from the bottom of my heart, i am so so sorry. i don't have words.