Police are responding to the scene now after reports of an explosion, saying in a statement that "there are a number of fatalities confirmed."
"Details of a casualty bureau will follow as soon as possible," the statement added.
Thousands of concert-goers can be seen fleeing the scene, following the reported loud bangs. Many attendees would have been teenagers and young children.
Fans recorded the chaos on social media, with one fan writing, "Just got out of Manchester Arena after seeing Ariana perform. There was a loud bang when the lights came on & everyone ran out screaming."
Ariana Grande's rep has confirmed to Billboard that the singer hasn't been harmed from the incident.
It's still unclear what the loud noises were, with organisers telling fans there is no need to worry and to exit the arena safely.
We'll update with further details as they come to light.
This article first published on Marie Claire.