When a group of Zenit fans published a December manifesto calling for sexual minorities and non-Europeans to be kept out of the St Petersburg club, they provoked condemnation across the world. Now, as Liverpool head to Russia for Thursday's Europa League tie, Zenit say they cannot guarantee an incident-free match.

Even within Russian football, which is notorious for entrenched racism, extreme nationalism and neo-Nazi elements, Zenit have a particularly bad reputation. Former managers have complained they were prohibited from hiring black players and there has never been a player of African origin in the squad.

Today, Zenit officials insist that any institutional racism is a thing of the past and that marginal beliefs are confined to a small group of fans. Russian law, however, means Zenit cannot exclude contingents of racist supporters from games, warned the spokesman Evgeny Gusev. "[Tackling racism] is not just the work of the club but of the legal authorities, the police and politicians," he said. "We work hard but we cannot be insured against individual cases."

The much-criticised manifesto, which made headlines around the world, was posted on the website of the fan group Landscrona, where it remains. Entitled Selection-12, it bemoans football's transformation into a money-making exercise and calls for local players and those from Slavic countries to be given priority at Zenit. "The absence of black-skinned players in Zenit's team is just an important tradition that underlines the identity of the club – and nothing more," it reads.

Landscrona has many fewer followers than Zenit's official fan groups, said one official close to the club who described the document as "a very naive text written by young people". But Grigory Yershov, who has been a Zenit fan for over a decade, said that the group, which puts on a football tournament for fans and has its own youth branch, enjoyed widespread sympathy. "One way or another probably every Zenit fan knows and respects Landscrona," he said.

Landscrona and the Russian Ultras, a grouping of hardcore and often violent fans, did not reply to requests seeking comment for this article. But Aleksei Rumyantsev, a prominent Zenit fan affiliated with Landscrona who had to go to hospital in August after being badly beaten up during a game with the Russian side Anzhi Makhachkala, denied all allegations of racism. And he drew a distinction between different sorts of fans, a distinction that is expressed in Russian by two different words: bolelshik, a simple supporter, and fanat, which is linked to the word for fanaticism. "There are fanati who do not have a culture of supporting [their club]," admitted Rumyantsev.

One of the most notorious incidents involving Zenit took place in March 2011 when an opponent, the high-profile Brazilian Roberto Carlos, was offered a banana. The club was fined £6,130. A fine was also levied in 2008 after fans threw bananas and made monkey chants during a game with Marseille. The France international Yann M'Vila reportedly declined an opportunity last year to move to Zenit following threats by some of the club's radical supporters. The infamous Zenit slogan, "There's no black in the colours of Zenit", can still be seen occasionally daubed on walls in St Petersburg.

The actions of Zenit fans has prompted concerns in Liverpool ahead of the Europa League game. Liverpool's managing director, Ian Ayre, said on Tuesday that he had written to Uefa and Zenit about the racism issue, and that his players would be advised on how to deal with abuse should it materialise. Liverpool have several black players in their team, including Glen Johnson, Raheem Sterling and Andre Wisdom.

Ever since Russia won the right to host the 2018 World Cup, increasing international attention has been focused on discrimination in the domestic game. Ethnic-fuelled violence between fans from Anzhi Makhachkala, based in the troubled North Caucasus, and Moscow and St Petersburg teams is common. "Racism is a habitual phenomenon for Russian football," said Natalya Yudina, an expert with the SOVA racism and xenophobia research centre. "But [the World Cup] means clubs will be forced to pay attention to their fans."

In recent years Zenit have been more vigorous in their efforts to change their image. The club have launched several anti-racism campaigns and, unlike his predecessors, their Italian coach, Luciano Spalletti, has said publicly that he is free to hire a player of any ethnic origin. The club broke their transfer record to sign the Brazilian striker Hulk and the Belgian Axel Witsel, whose father is from Martinique in the French Caribbean, for a combined £64m in September. Despite a dressing-room walkout by the Zenit captain, Igor Denisov, who refused to play in protest at the high salaries being paid to the new players, Hulk said this month that he has never experienced any racism at Zenit.

Russian law limits the options available to a club in dealing with incidents on the terraces and prevents the exclusion of racist fans and other hooligans from matches, according to Zenit officials. But new legislation, which is expected to be passed by the summer, will give clubs more power to fine troublemaking fans and ban repeat offenders. The proposed changes were given added impetus after an incident in November when Dynamo Moscow's goalkeeper was hit by a flare thrown from the Zenit stands, forcing the match to be abandoned.

Zenit have no shortage of powerful backers. Their most prominent supporter is the prime minister, Dmitry Medvedev, who, like many of the Kremlin elite including the president, Vladimir Putin, is a St Petersburg native. The club's official sponsor is the state-owned gas giant Gazprom, which is funding the construction of a new stadium that is slated as a 2018 World Cup venue.

Not all agree that racism is an issue in Russian football. "Zenit supporters have never made any directly racist statements," said Alexander Shprygin, a member of the executive committee for the Russian Football Union, the sport's national governing body. "Russia does not have a problem with racism. What happens in Europe is much rougher and more cruel."

Many argue that racist incidents have been blown out of all proportion and that Liverpool and other visitors have nothing to fear. The manifesto, Selection-12, published by the Zenit fans was open to different interpretations, said the commentator and football expert Andrei Malosolov. "You could call it racism, but you could call it a fully sensible proposal," he said. Malosolov added that players from ethnic minorities who complained about discrimination, or who performed badly on the pitch, were the ones likely to be targeted with racist abuse.

"I am not a racist but I would like Russian people to play for my club," said Denis Kostamarov, 27, a supporter of Zenit for 15 years. "If it's a very talented player … we are only too happy if they come and play football for Zenit. But to buy a black player just to say we are not a racist club is stupid."