The former Premier League footballer Colin Kazim-Richards was found guilty in a landmark case on Wednesday of making an "utterly disgusting" homophobic gesture at Brighton and Hove Albion fans during a football match last year.

The former Blackburn Rovers player, 27, denied the charge and claimed he was bantering with the crowd. But he was convicted of making the gesture to fans of his former club on several occasions during a Championship match on 12 February last year. He was fined £750 and ordered to pay costs which amounted to £1,445.

In the first trial of a footballer accused of making a homophobic gesture, Brighton magistrates court heard from four witnesses who said Kazim-Richards had made a masturbatory gesture next to his backside after receiving abuse from the home crowd.

Darren Hastings, a season ticket holder and lifelong fan of Brighton and Hove Albion, said a friend had alerted him to the gesture Kazim-Richards allegedly made. "He leaned behind him and pretended to insert something in his bottom," he told the court. "I was utterly disgusted, I am a season ticket holder I understand football players receive a number of gestures or comments from fans but I was clearly shocked to see a football player perform a gesture like that on a field of play."

Retired police detective Michael Beard, also a season ticket holder at the Amex Community Stadium, said Kazim-Richards had been singled out for abuse. He described the footballer putting his hand behind his back, bending his knees and simulating pushing something towards his behind, while slightly thrusting his pelvis forward. "I thought it was a clear homophobic gesture in reaction to the chants of the crowd," he said. Asked by Simon Allen, prosecuting, what he thought of the gesture he said: "I was saddened by it actually."

Stan Reiz, defending. said the footballer had been subjected to chants including "you fat bastard" and "you are fat, and you know you are". He questioned witnesses about homophobic chanting at Brighton and, in his closing speech suggested that the gesture was a matter of perception with Brighton fans "sensitive" to perceived homophobia because of abuse they had previously received from visiting fans.

Kazim-Richards admitted making a "wanker gesture", but said it was behind his back so the referee did not see him and it was only "banter" with fans. "They were doing it to me and I was giving it back to them," he said. He denied that the gestures were intended to be homophobic or offensive.

Kazim-Richards, who now plays for Turkish club Bursaspor after stints at Sheffield United, Fenerbahce and Galatasaray, described being singled out for abuse by one woman with a piercing voice. "I was getting called, excuse my language, you fat bastard," he said, but added that he was "not at all offended".

In the latter part of the game he had made two gestures, he said. "I was interacting with the fans, doing basically what they were doing to me, having a bit of banter back." Asked if he had made a homophobic gesture, he said: "At no time did I do as the witnesses described."

The court heard that Kazim-Richards had been an ambassador for the FA's Kick It Out campaign that seeks to eliminate discrimination in football. "I don't agree with discrimination in any form or shape," he said.

Sentencing Kazim-Richards, magistrate Darren Reynolds said that each of the witnesses had been very sure that the gesture made was both homophobic and offensive, and had maintained that position despite "robust" cross-examination. "Each was clear about what they saw and we accept their account," he said. "We do not accept your account ... You used insulting behaviour in a situation that someone was likely to find offensive."

Outside the court, Darren Balkham, football liaison officer for Sussex police, said it was a historic case which would warn others that homophobic abuse on the pitch or on the terraces would not be tolerated. "Brighton fans have been subjected to this abuse over many years," he said. "We are where we were 20 years ago with racism. Racism has largely been dealt with but homophobia still in some places seems to be a grey area. You wouldn't do it in an office, why do it on the terraces?"

Balkham added that footballers who made similar gestures would be prosecuted. "We are taking steps forward but society, schools and parents also have to say because someone is different doesn't mean that is bad ... We want to make sure football is inclusive for all."