There have been reports of "explosions" and panic at the Manchester Arena in the UK, where Ariana Grande was performing.
UPDATED 12.21pm: Police have just confirmed 19 dead with around 50 more injured.
UPDATED 11.58am: Police have confirmed via twitter there has been fatalities in the incident.
"Just got out of Manchester arena after seeing Ariana perform. There was a loud bang when the lights came on & everyone ran out screaming," one person wrote on Twitter.
Concertgoer Hannah Dane told the Guardian there was "quite a loud explosion heard from inside the Manchester arena and it shook, then everyone screamed and tried to get out".
"As we got outside, lots of police came racing towards the area and the whole of the Victoria train station was surrounded by police."
There were people "screaming and crying everywhere shouting that there's a bomb and also people were saying there's a shooter," she said.
Police are warning people to stay away from the area, and emergency services are rushing to the scene.
A woman who lives opposite the arena told the Press Association she "just heard a huge bang from my bed, came out to the front of my apartments 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," Suzy Mitchell said.
"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."
An Ariana Grande fan Twitter account says security did not check anyone's bags, and claims a bomb went off. The Twitter account has shared an image purporting to show bodies on the ground at the scene, but it has not been verified.