Inter Milan were ordered to play their next two home games behind closed doors on Thursday after their supporters racially insulted Napoli defender Kalidou Koulibaly during Wednesday's Serie A match at San Siro.
Koulibaly was also handed a two-match ban by Serie A's disciplinary tribunal after he was sent off in the 81st minute for sarcastically applauding the referee for booking him.
Napoli said on their club website that the Senegalese player was subject to "racist chants" during the match, without giving further details, while their coach Carlo Ancelotti said some fans made animal noises throughout the entire game.
In addition to the next two home matches against Benevento, in the Italian Cup, and Sassuolo in Serie A being staged behind closed doors, the section of the San Siro stadium occupied by Inter's "ultra" fans has also been ordered to be kept closed for a third game in the league against Bologna.
The mayor of Milan, Giuseppe Sala, apologised to Koulibaly over the insults, describing them as "a disgrace" on his Facebook page.
"It was a shameful act against a respected athlete, who proudly bears the colour of his skin, and also, to a lesser degree, against the many people who go to the stadium to support their team and be with their friends."
Napoli lost 1-0 after conceding a stoppage-time goal, leaving them nine points behind Serie A leaders Juventus. Like Koulibaly, their forward Lorenzo Insigne was also given a two-match ban after his sending off in stoppage time.
Koulibaly was booked for fouling Matteo Politano and then dismissed for his reaction to the decision, although Ancelotti said the player was already upset and on edge over his treatment.
"I'm disappointed by the defeat and above all to have left my brothers," said Koulibaly on Twitter. "But I am proud of the colour of my skin. Proud to be French, Senegalese, Neapolitan: a man."
The FARE network, which monitors discrimination in European football, said it was a familiar tale.
"Once again in Italian football. Player is racially abused, referee fails to act, player is angry and gets sent off. The same cycle again and again," it tweeted.
Ancelotti said Napoli asked an Italian federation (FIGC) delegate three times during the match for the game to be suspended but, instead, public announcements were read out asking fans to stop.
The coach said Napoli would walk off the pitch if there was a similar incident in the future.
Inter Milan coach Luciano Spalletti added that it was the sort of behaviour which was holding back Italian football.
"If 65,000 people come and watch the match at Christmas, they want to see something else," he said. "We need a change of mentality (if) our objective is to bring our football back to the top in Europe."
Match officials in Italy are expected to report racist incidents to public security officials who have the power to stop games.
The guidelines were introduced in 2013 after the AC Milan team walked off the field during a friendly match in protest at racist insults aimed at several of their players.
Last year, there was an outcry after Ghanaian player Sulley Muntari was booked and sent off after complaining about racist abuse while playing for Pescara at Cagliari.
Muntari, who said that he was booked for asking the referee to stop the match, walked off the pitch in protest and was then given a second yellow card for leaving the field without permission.
Cristiano Ronaldo, currently the most celebrated player in Serie A, was among those quick to offer supportive messages to Koulibaly on social media.
Juventus's Portuguese striker posted a picture of himself playing against Koulibaly with a message in Italian: "Education and respect are needed in the world and in football. No to racism and to any offence and discrimination."