"We could hear the police and ambulance sirens. It was terrifying.
"There were thousands of people trying to get out at once. They were all screaming and crying. The whole place smelt smokey and burnt.
"The explosion sounded like it was inside the building somewhere."
Manchester Victoria train station, next door to the arena, is currently closed and trains are currently unable to run to or from the station.
Greater Manchester Police said emergency services had responded to a 'serious incident' at the arena.
Jonathan Yates, 24, from the Wirral, who attended the gig, told MailOnline: 'The concert had finished and the lights came on almost instantly. There was a bang, a weird bang. There were lots of balloons but I thought to myself that can't be a balloon, that's not normal.
"We were sat on the floor level and it came from the higher seats, people were running and screaming. Everyone stopped and I turned to my friend and said 'we need to run'.
Everyone was running and screaming and then when we got out it felt a bit more OK.
"I heard five or six bangs that sounded like gunshots. When we got outside people were outside, crying and on their phones.
"You don't think it's something that's going to happen when you go...it was such a nice, fun concert."
Nick Schurok, 28, from Manchester, told MailOnline: "Ariana Grande had just finished the concert and the lights came on. Everyone started leaving. I was on the floor and at the back of the arena people started exiting through the tunnels.
"There was a bang in the left tunnel and everyone went to the middle tunnel. Then about two minutes later, there was another bang. The bang was so loud and crowds of people were running. There were lots of children and families there."
Another witness Jenny Brewster told MailOnline: "We were exiting the building when it happened. We'd headed towards the main doors as Ariana was performing the last song because we wanted to beat the crowds, but - as we made our way there - a wall of security men blocked it and told us to go the other way.
"Seconds later they shouted 'RUN!' and the explosion happened right behind them. Hundreds of people were running and screaming. Those men saved our lives."
Catherine Macfarlane told Reuters: "We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming.
"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 of the area."
One witness wrote on Twitter: "Just ran from an explosion, genuinely thought we were gonna die."
Another said: "Explosion at Manchester Arena, we were evacuated, a LOT of police here."
Former Manchester United footballer Rio Ferdinand said: "Just heard the news what's happening in Manchester.. hope everyone safe and sound!"
A spokesman for Greater Manchester Police said: "Emergency services responding to serious incident at Manchester Arena. Avoid the area. More details will follow as soon as available."
A British Transport Police spokesman said: 'We are aware of an incident at Manchester Arena. We have officers at the scene and will provide further updates as soon as possible.'
