Fifteen people treated after toxic gas released in Melbourne nightclub

At least one person has been taken to hospital after a toxic substance was released on the dance floor of a Melbourne nightclub overnight.

Police were called to the Perseverance Hotel at Fitzroy about 1am on Sunday, following reports of an assault.

But when they arrived, they discovered a medical emergency unfolding outside.

Fifteen people had to be treated by paramedics after they inhaled the mystery substance.

Staff have told Seven News it was a "difficult" night.