Lille said two fans were stabbed in a “cowardly and unbearable attack” before Wednesday's Champions League game against Bologna.
The Ligue 1 club urged fans to “exercise vigilance” after learning of the incident.
According to Italian media, citing local police, around 80 people, believed to be Bologna supporters, attacked about 30 Lille fans outside a bar in the city's university district, armed with metal bars and belts.
In total, three Lille fans were injured and treated in hospital before being released, police said.
Lille said in a statement: “The club unreservedly condemns this cowardly and intolerable attack, which goes against all the values of football and popular support.”
“We offer our support to the supporters who are victims of these attacks and wish them a speedy recovery.”
Lille is fighting for a place in the Champions League play-off round of 16 – it sits 17th in the 36-team table with seven points – while Bologna has a single point after four games and is battling elimination.