Forty people were treated by paramedics after an irritant - suspected to be CS spray - was reportedly released inside a nightclub.
Dorset Police said revellers complained of shortness of breath and stinging eyes following the incident at Cameo in Bournemouth.
One person was taken to hospital as a precaution.
Officers were called to the club in Fir Vale Road at 2am on Saturday, and management evacuated the venue.
Pictures on social media show crowds of clubbers gathered in the street outside as police and ambulances attended the scene.
Josh Wilde, 20, told the Press Association the club had recently introduced thorough searches on the door with airport-style scanners, but he said only some clubbers had to go through them.
The third-year journalism student said: ‘It had the most security at a club I've ever seen outside central London. There were hundreds of people queuing to get in, it was absolutely packed, I queued for at least an hour.
‘I initially thought someone had got on to the stage because security were running around the main room, but then a firefighter rushed past in full gear so I knew it must be something more serious.
‘Outside people were coughing, someone told me all their friends just started spontaneously coughing and crying. There were around two to three ambulances and six or seven police cars and a lot of officers.
‘Seeing the size of the emergency response brought it home that something serious must have happened.’
A Dorset Police spokeswoman said: ‘At 2am today, Dorset Police received a report that some type of irritant, possibly CS spray, had been released within the Cameo nightclub on Fir Vale Road, Bournemouth.
‘Following a decision by management of the premises to evacuate, around 40 people were treated by the ambulance service for minor effects, such as shortness of breath and stinging eyes.
‘One person was taken to hospital as a precaution.’
The force said officers had searched the nightclub and found no evidence of any chemical or noxious substance which posed a continuing risk to the public.
Rapper Yungen had been performing at the club as part of an event to mark university freshers' week.
He later said on Twitter: ‘Everyone that just come to see me in Bournemouth as I'm sure all of you could see the police shut off my mic without saying anything mid performance because apparently someone sprayed pepper spray ... I hope everyone's safe’.
The investigation into the incident is continuing.