British authorities are treating the explosion as a "possible terrorist incident." There are reports it was a suicide bomber, and he used a nail bomb to inflict more damage.
The Manchester Arena tweeted that the explosion happened outside of the arena in a public space.
Videos and photos of panicked fans fleeing the arena have made their way to Twitter, and several eyewitnesses claim they saw blood and smoke in the aftermath of the said explosion. TMZ reports that police descended on the area and urged people to stay clear of the area. Greater Manchester police tweeted, "There are a number of confirmed fatalities and others injured."
"Details of a casualty bureau for incident at Manchester Arena will be shared as soon as available. Please stay away from the area," GM police added.
Multiple law enforcement sources in both Britain and the United States told NBC News that at least 20 people were killed at possible "hundreds" of others injured after one or more loud bangs were heard. Billboard confirmed that Grande is "okay," and the singer's team is currently investigating what happened.
A Twitter user who was posted near the venue claims to have recorded an audio of the explosion right when it occurred.
An eyewitness named Andy Holey told the BBC (via ABC News) that he was waiting outside the arena when he was blown over by a forceful blast near the box office. Holey also told the outlet that he saw many casualties around him but was unsure if they were injured or dead.