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"The cause may have been the dispersal of a stinging substance, the young people fled and trampled over each other. Sadly, six people died and dozens are injured," the fire service said in a statement on Twitter.
Local reports said around 1,000 people were in the Blue Lantern club in the town of Corinaldo on the Adriatic coast for a performance by Italian rapper Sfera Ebbasta.
The accident happened around 1:00 am (0000 GMT).
"We were dancing and waiting for the concert to start when we smelt this pungent odour," a 16-year-old boy who was taken to hospital told the media.
"We ran to one of the emergency exits but we found it blocked, the bouncers told us to go back."