Police were called to a fight at a hotel after the staging of an event involving boxer Mike Tyson.
Derbyshire Police said they received an emergency call at 11.35pm that "a large fight" was taking place at The Heritage Hotel in Macklin Street, Derby.
When police arrived, there was no fighting, said the force, but four people had suffered minor injuries. One was taken by ambulance to Derbyshire Royal Infirmary, but has since been discharged.
An appeal for witnesses has been launched.
Tyson's first date in a series of dinner functions was last night at The Heritage, where former rival Frank Bruno was billed as a special guest. Other dates are planned for Birmingham, Manchester and London.
Derbyshire Police were unable to confirm any more details about the incident and who was involved.
The men involved in the brawl were spectators at the event, Derbyshire Police confirmed this morning.
A force spokeswoman said: "The VIPs had gone. It was just spectators who had got involved in the fight. Nobody involved has made any complaints. The 999 call was made by the hotel."
She added that a number of brawlers refused to be taken to hospital by ambulance.
"Several turned up by themselves but decided they did not want any treatment when they got there," she added.
The injuries, she said, were mainly cuts and bruises.
Around 20 people were involved in the brawl, which saw chairs hurled, tables tipped over, glasses smashed and punches thrown.
It reportedly began during an auction of boxing and Tyson-related memorabilia.
Tyson and Bruno had answered questions from the 500-strong crowd after a meal and left before the trouble started. The Heritage Hotel declined to comment on the incident.