Lil Pump's Nottingham gig was cut short as fans had to flee to safety after a suspected smoke flare was set off during the US rapper's performance.
Police said at least four people were being treated for minor injuries following the incident at the Rock City venue on Friday night.
In a statement, the music venue said it may have been pepper spray that had been let off while some concertgoers speculated that tear gas was let off at the show.
Nottinghamshire Police said they were called to "what is thought to be a smoke flare or similar having been set off inside the venue" and that they had now launched an investigation.
The force added: "The venue has been evacuated and officers are assisting with this to ensure public safety.
"Our medical and fire colleagues are on scene and are treating a number of people for minor injuries."
Rock City said it was not aware of any "serious casualties" so far.
"Putting the safety first of everyone attending tonight's show, we've taken the decision to end the show as it is suspected that pepper spray was let off inside the Main Hall," the venue said.
"We evacuated the building and are currently working with the fire service to ensure the building is safe to re-enter and that anyone who may have been affected got immediate treatment."
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The surrounding area was cordoned off by officers as emergency services attended the incident.
Video on social media later emerged of the Gucci Gang rapper performing for fans on top of his tour bus outside the venue.