The Italian Football Association has launched an immediate inquiry into alleged racism by supporters at a friendly match involving AC Milan.
The game at fourth-tier side Pro Patria was halted in the first half when Milan's midfielder Kevin-Prince Boateng kicked the ball at the stand in protest and walked off the pitch, soon followed by his team-mates.
Italian FA president Giancarlo Abete branded the incident "unspeakable and intolerable" and an offence to Italian football and announced the incidents would be immediately investigated.
Piara Powar, executive director of European anti-discrimination group FARE, demanded strong action from the Italian governing body.
He said: "We salute Kevin-Prince Boateng for his actions and his team-mates for their support. This is the not the first time a player has walked off in Italy - if the situation continues it may not be the last.
"Italy, as much as any country in Europe, has a serious problem of racism to deal with. Football infrastructure is in need of renewal and at serious odds with the changing nature of Italian society. We look forward to strong action by the FIGC (Italian FA)."
Milan's website said there was also racist abuse by "a minority of Pro Patria fans" at players Mbaye Niang, Urby Emanuelson and Sulley Muntari.
Milan coach Massimiliano Allegri said: "We are disappointed and saddened by what has happened. Milan play for the right to respect all players. We need to stop these uncivilised gestures.
"We are sorry for all the other fans who came here for a beautiful day of sport. We promise to return, and we are sorry for the club and players of Pro Patria, but we could not make any other decision.
"I hope it can be an important signal. Italy needs to improve & become better educated & more intelligent."
Many footballers voiced their support for the actions of the Milan players.
Patrick Vieira, who played for Inter Milan, tweeted: "AC Milan are an example to all. It's a strong and bold statement to make by the club and players."
Manchester United's Rio Ferdinand tweeted: "If the stories about KPB walking off the pitch with teammate's after being racially abused are true,fair play to him..well done#UefaStandUp"
While Manchester City's Vincent Kompany said: "I can only salute Milan's decision to leave the pitch. Also noted that the majority of the fans were completely supportive of the players."