Jonathan Yates, who attended the gig, said: "As the Ariana Grande concert finished there bangs like gunshots - it was terrifying so many people running and crying."
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."
A spokesman for Greater Manchester Police said: "Police responded to reports of an incident at Manchester Arena. Please stay away from the area. More details to follow..."
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."
- This article was first published on nzherald.co.nz and is republished here with permission.