Up to 500 Croatia fans racially abused Italy forward Mario Balotelli during their countries' Euro 2012 game, an anti-racism monitoring group backed by European football's governing body said on Friday.
The executive director of Football Against Racism in Europe (FARE), Piara Powar said their observers had reported the matter to UEFA and that an inquiry will be launched.
"We have got two observers at every match and we make sure that they speak the language of the two teams involved," he told AFP.
"Our observers reported between 300-500 Croatian fans were involved in racist chanting in the match against Italy. That is a big number. It was fairly intermittent but progressive throughout the game. It intensified when Balotelli was substituted.
"Our Italian observer is a fairly seasoned football fan and said he was shocked by what he had seen and heard. We have given a report to UEFA and they will now open an inquiry. We are looking at what other evidence there may be."
An AFP photographer positioned in front of the majority of Croatia fans during Thursday's match, which ended 1-1, earlier said that he heard monkey noises being directed at the Manchester City striker.
At one point, he also saw a steward picking up a banana that had been thrown onto the pitch. Balotelli, 21, has previously said that he would walk off the pitch if he were racially abused by fans.
UEFA had no immediate comment when shown the AFP photo of the incident and was unavailable to respond to FARE's claims. But a spokesman has said they were awaiting the match report from English referee Howard Webb.
A spokesman for the Italian football federation meanwhile said that neither Balotelli nor any other player had mentioned either being abused or the throwing of a banana, although officials were unhappy at booing during the national anthem.
UEFA has said it was investigating claims Balotelli, who is of Ghanaian origin, was racially abused during Italy's 1-1 draw with Spain in Gdansk on Sunday as well as similar allegations involving Russian fans and Czech player Theodor Gebre Selassie.
Selassie is of Ethiopian origin.
Italy coach Cesare Prandelli, though, had strongly rejected the claims in a number of British newspapers that Balotelli was subject to racist taunts against Spain.
UEFA president Michel Platini said at a pre-tournament news conference in Warsaw that if Balotelli did that, he would receive a yellow card, advising him instead to alert the match referee, who can now stop matches in the event of racist abuse.
Although UEFA has said it has opened an inquiry into the previous claim against Balotelli and Gebre Selassie, it has not started any formal disciplinary proceedings.
Dutch players have also complained of racist taunts during a training session in Krakow but no formal complaint has been made by the Dutch football federation.
Racism has become a major issue for tournament co-hosts Poland and Ukraine after claims in a BBC television documentary that far-right extremist gangs were rife at football grounds in both countries.
The British government, among others, issued a warning to black and ethnic minority fans intending to travel to Euro 2012 about possible racist attacks, which prompted the families of two black England footballers to stay at home.
Warsaw and particularly Kiev have strongly denied the allegations, with both they and UEFA maintaining that racism is a problem across European society and not just at football grounds in the two eastern European countries.
Croatia were on Friday separately fined 25,000 euros ($31,400, 20,000 pounds) as fans lit flares and let off smoke bombs to celebrate goals in their opening match against the Republic of Ireland, also held in Poznan, UEFA announced.
Missiles were also thrown and a spectator got on the pitch.