Italian football is staring into the abyss again after a brief recovery from a match-fixing scandal that saw Juventus relegated in 2006. This time the malaise follows a rash of allegations of match-fixing that may involve more than half of the teams in Serie A, as well as the return of hardcore fan violence.

The violence reached a peak last weekend when extreme ultra fans of Genoa, watching their side go down 4-0 to Siena, became so enraged they broke into the family stand, threw firecrackers on to the pitch and climbed on to the roof of the players' tunnel before demanding that the players take off their shirts, claiming they were unworthy of wearing the team colours.

Amazingly, as play stopped and the police stood by, most players obliged, with enraged fans yelling: "We want your socks, we want you in your underpants." In a further twist Giuseppe Sculli, the only player who was brave enough to keep his shirt on and negotiate with the ultras, says he attributes his courage to his grandfather – who was a notorious mafia drug lord.

As ultras continue to run the terraces – the antisemitic abuse hurled by Lazio fans this month was barely noticed – regular supporters are abandoning Italy's stadiums. Attendances fell between 2008 and 2010 by almost 300,000 across the top three leagues, while in Serie A average gates are just 23,000, far short of the 35,000 in the English Premier League.

Enrico Preziosi, the chairman of Genoa, admitted after his players' striptease that there was "widespread fear" of ultras, as fans at Atalanta and Milan unfurled banners backing their Genoa counterparts, the latter warning Preziosi "not to judge the ultras". The banner was a reference to Preziosi's punishment for buying a crucial match against Venezia in 2005, a scam that was rumbled when police nabbed a Venezia official with €250,000 (£204,000) stuffed in a bag.

Match-fixing is set to return to the headlines next month when Italy's football federation will order around 50 players to go before a sporting tribunal. Last year wiretaps handed over by prosecutors in Cremona led to 17 players, including former national team stars Giuseppe Signori and Cristiano Doni, being suspended.

The federation is rushing to mete out justice as prosecutors in Bari and Naples investigate match-fixing on their own patches, notably a crucial Bari-Lecce derby last season during which Bari defender Andrea Masiello has admitted scoring a deliberate own goal to ensure his side's defeat and a €50,000 payoff.

Maurizio Galdi, who follows football corruption for sports daily La Gazzetta dello Sport, said up to 12 Serie A sides, representing more than half the division's 20 teams, could be implicated. "About 75 matches since 2009 are now being investigated and at the end of all this we could see around 50 players sent for criminal trial," said Roberto Pelucchi, who also writes for the paper.

While Masiello is suspected of being paid to help Lecce avoid relegation, most matches were allegedly fixed to win bets, often arranged by a Balkan gang known as "the Gypsies". In Naples, the Camorra mafia are also suspected of involvement.

"When caught, players deny everything, including the Naples goalkeeper who claimed innocence for hours until he was made to listen to a wiretap of himself offering to distribute bribes to teammates," said Galdi.

"Players in lower divisions may bet on fixed games because they haven't been paid for six months, while players in Serie A think they are gods who can make a match end whichever way they want."

In a bid to stamp out match-fixing next season, players will stand before fans and swear loyalty to their sides. But one commentator said that fans might stick by their teams regardless because they were so inured to corruption.

"Even if the game is riddled with gambling, corruption, bad refereeing and secret agreements, well, in Italy everyone is at it," wrote political scientist Ilvio Diamante. "Everywhere. In politics, business, at work. Why be scandalised? That's life."