This weekend sees Lazio take on Roma and Liverpool host Manchester United (as discussed in Best of Enemies and A Rivalry in Deepest Red). Here, we look at some of the most hate-filled rivalries in world football.
Bulgaria's Eternal Derby (CSKA Sofia vs Levski Sofia)
With its roots in the 1940s, many see Bulgaria's Eternal Derby as the biggest rivalry in the Balkan region: CSKA, founded as the Communist army team and associated with the rich, against Levski, seen as the team of the people.
On the field, the fiercest battle came in the 1985 Bulgarian Cup final, which saw two red cards, numerous fights and the referee struck twice by goalkeeper Bobby Mihailov. Tempers were sparked when CSKA's Georgi Slavkov opened the scoring in the 26th minute via a handball, and that goal proved decisive in a 2-1 victory. At the end of the game, a huge brawl took place in the tunnel that was severe enough to see the Bulgarian Communist Party order that both teams were dissolved while five players, Hristo Stoichkov among them, were handed life bans. A few months later, the sanctions began to be reduced, allowing the players to play on and the clubs to live on.
The off-field violence has only increased, though, particularly after Communist rule was overthrown in 1989. "It's like a war now," one Levski fan told Reuters in 2008. "I'm a lifelong Levski supporter and I've been at the stadium for more than three decades but it seems ridiculous to go there with my kids nowadays. It's so dangerous there."
Cairo Derby (Al Ahly vs Zamalek)
Not only the most successful teams in Egypt but the most successful in Africa, the Cairo Derby sees a vicious division between the red of Al Ahly - the team of the nationalistic proletariat - and the white of Zamalek - the team of the colonial bourgeoisie.
The derbies, which have crammed in 120,000 fans, have long been some of the most violent in the game despite the armed forces' efforts to police them. In 1966, a controversial penalty saw riots at Zamalek and the burning of a local factory while, in 2007, with the violence shifting to the basketball games, a Zamalek fan was set on fire after Ahly fans hurled Molotov cocktails. Foreign referees were brought in and the games were moved to the neutral Cairo International Stadium to help ease the tensions, but the problems have not abated.
In a 1999 derby, French referee Mark Batta sent off Zamalek's Ayman Abdel Aziz after just two minutes for a tackle from behind, prompting his team-mates to walk off in protest. Zamalek were fined by the Egyptian FA and saw their coach and two players suspended, before being sued by three Zamalek fans for £180,000 in 'moral damages'. A lawyer then chipped in attempting to sue the club and federation for £18,000, plus his £180 travel costs and £2.75 match ticket. Such is the passion of the Cairo Derby.
The two clubs' ultras actually united as they fought in the 2011 Egyptian revolution, but only briefly. "For a few hours," one Ahly fan told CNN, "but I couldn't do it for long."
De Klassieker (Ajax vs Feyenoord)
The clubs are separated by 43 miles, but they have long been arch-rivals. Ajax are the Total Footballers, the club of cultured and liberal Amsterdam, while Feyenoord are the Rotterdam club, associated with the right-wing working-class.
Feyenoord fans direct anti-Semitic chants at their rivals, while Ajax describe themselves as the 'Super Jews' and decorate the ground with the Israeli flag and Star of David, but as one Ajax season-ticket holder and son of a Holocaust survivor told The Times in 2007: "We're not a Jewish club at all. It's just these bloody kids. They just want some sort of identity, but it's insulting."
Both sides have a serious history of violence. In 1996, Feyenoord fans firebombed police and threw stones and bottles at the Ajax team bus; the following March, in a pre-arranged battle that became known as the Battle of Beverwijk, an Ajax fan was beaten to death as fights involving knives, baseball bats and hammers caused havoc.
The 1997 incident prompted politicians to speak out about the "madness" and to seek solutions, but they have been unable to stop the violence. In 2004, Ajax fans flooded the field at the end of a reserve match, leaving Feyenoord midfielder Jorge Acuna with head, neck and rib injuries as Robin van Persie made a narrow escape; in April 2005, there was a notable riot in which seven police were wounded.
Derby de Casablanca (Raja vs Wydad)
The Casablanca derby sees two of Morocco's big three battle it out in front of crowds of up to 80,000 in a rivalry with a long history that has grown ever stronger down the years.
Wydad's early years are tied to former journalist Affani Mohamed Ben Lahcen, or 'Pere Jego', who helped found the club in 1937 and became its first coach. Such was his success - he won four domestic league titles, three North African titles and one North African Cup - that he apparently faced two assassination attempts. He fell out with the Wydad directors and left the club in 1952; three years later, he turned up at Raja, who had been formed six years earlier.
Pere Jego was a well-travelled coach and the timing of his trips around the globe lay the foundations for each club's characteristic approach. Wydad, the middle-class club, had been schooled in the English style of play; Raja, the working-class side, were educated in the Brazilian style.
The rivalry has led to regular rioting in the stands and Moroccan magazine TelQuel recently wrote of a "veritable field of battle" that "does not have very much to do with sport" after a match-day in which one fan was killed, 150 buses were vandalised, police vehicles were damaged and the Mohammed V stadium was severely damaged. "These hooligans are just extremists," Raja president Abdellah Ghallam said. "The thugs here don't even look at what's happening in the game."
Derby della Capitale (Lazio vs Roma)
Though the Milan derby may attract more global attention, Italy's fiercest contest concerns its capital clubs. Roma were traditionally the people's club, founded in a 1927 merger between three Rome-based sides; Lazio, associated with the well-heeled, remained independent thanks to the efforts of Giorgio Vaccaro, a general in the Fascist regime. "Lazio is different," Vaccaro explained. "Lazio came first, then its fans. As for the others, the fans were already there and they were handed a team to support."
Benito Mussolini was to become heavily associated with Lazio, but the regime was keen to see both Rome clubs prosper - in fact, Roma's title triumph in 1942 was attributed to the dictator and Helenio Herrera, who took charge of Roma from 1968, later gave that theory credence when he said they "won their lone Scudetto thanks to Mussolini". Even so, the battle to become the capital's foremost club intensified the rivalry.
The derby made international headlines in 1979 when it bore witness to the first violence-related death in Italian football: an 18-year-old Roma supporter fired a flare across the pitch, killing Lazio fan Vincenzo Paparelli.
The hatred has continued, with Lazio's Fascist contingent attracting attention in recent times. They memorably unveiled large banners reading "Auschwitz is your home, the ovens are your house" and "Black team and Jew fans" in 1998 and 2001 respectively, although the Jewish Telegraphic Agency reported that, during the '98 derby, the "Roma fans in turn displayed an anti-Semitic sign that used Nazi and Holocaust imagery". Derbies in 2004 and 2009 were respectively abandoned and suspended due to crowd problems.
Derby of the Eternal Enemies (Olympiakos vs Panathinaikos)
The fiercest derby in Greece has the classic characteristics: Olympiakos and Panathinaikos are the country's two most successful clubs and, in their early years, represented different social strata. Panathinaikos, founded in 1908, were considered to embody upper-class society; Olympiakos, founded in 1925, were the team of the working-class and became symbolic of the proletariat struggle.
Though the class divisions have eroded, the rivalry is as fierce as ever and extends across a range of sports including basketball and water polo. It was prior to a meeting of the sides' women's volleyball teams in 2007 that a 22-year-old Panathinaikos fan was stabbed to death in a pre-arranged meeting of around 300 hooligans, and subsequent police raids on supporters' clubs uncovered pickaxes, iron bars and baseball bats. The Greek government subsequently suspended all professional sports in the country for two weeks.
El Clásico (Barcelona vs Real Madrid)
El Clásico has long been one of the world's most compelling rivalries. With one of the duo having been crowned domestic champions in 52 of the 80 seasons since La Liga's foundation in 1929, they are by far the most successful clubs in Spain.
It is political divisions, though, at the heart of the hatred: Madrid became associated with General Franco's dictatorship, while the support of Barcelona became a legitimised expression of Catalan individuality - hence the famous més que un club motto.
Real Madrid's signing of Alfredo di Stefano in 1953 - the leading light as Los Blancos dominated the early years of the European Cup - stoked tensions further. Both clubs had agreed to sign Di Stefano from Millonarios; when it was decreed that he would play two seasons for each club, Barca - in circumstances that are still not entirely clear to this day - withdrew. The Guardian, in a 1954 editorial piece on Franco, wrote: "Up to now, the political discontent had been diverted to the bull-ring or the football field. To-day the football field is adding to the political discontent ... As a result of [the Di Stefano affair], Catalanism, which no one wanted to see perpetuated, flamed up in Barcelona into a vigorous new life."
In recent years, sporting issues have added fuel to the fire. Luis Figo's move to Madrid - pig head and all - is well documented, while the current pre-eminence of the duo and the arrival of Jose Mourinho have ensured pitched battles are now de rigueur.
Eternal Derby (Partizan Belgrade vs Red Star Belgrade)
Known simply as 'the derby' in Serbia, there was a fierce sporting rivalry between the Belgrade clubs until the mid-1980s and, from then on, it grew more and more violent. In the 1990s, attendance was considered risky. Since 2000, it has been - one Partizan fan tells ESPNsoccernet - "no place for a sane person to go".
Red Star were European champions in 1991 but within months the war had seen them suffer a heavy decline that their president, Vladimir Cvetkovic, labelled "a tragic parable of the disaster in this country, one of the many victims of the madness". The decline has persisted, Partizan also affected, with the systematic problems in the Serbian game ensuring any young players of promise are quickly sold off for the profit of often questionable owners.
Kıtalar Arası Derbi (Fenerbahce vs Galatasaray)
They have been mortal enemies for over 75 years, Istanbul rivals whose fans have celebrated victories with guns and fought with knives as a matter of course.
The animosity began with a match in the 1933-34 season, a fierce battle that saw hard tackles escalate into an outright battle between the players and, eventually, the fans. The referee was forced to abandon the game after an hour, and 17 players - nine Fenerbahce and eight Galatasaray - were handed long-term bans.
The rivalry has remained intense ever since, and was memorably inflamed during Graeme Souness' solitary season in charge of Galatasaray. The former Rangers and Liverpool boss, under pressure after a disappointing season, celebrated a victory in the second leg of the Turkish Cup final by planting a Galatasaray flag in the centre-circle of Fenerbahce's pitch. Violence erupted as the fans ran to rip out the flag and Souness raced down the tunnel. One Turkish columnist, Hıncal Uluç, wrote that it "could have been another Heysel", though Souness played it down in bizarre fashion: "We always do that in Britain. I can't understand why the Fener fans acted so strongly."
If he had not grasped the depth of the hatred, an incident in December 2010 made it clear: 13 Fenerbahce players were hospitalised when set upon by Galatasaray fans during an under-17 match.
Old Firm (Celtic vs Rangers)
Since the foundation of the Scottish First Division in 1893, Celtic and Rangers have dominated Scottish football. Teams have occasionally threatened the big two with periods of success spanning a number of years - Hibernian after the Second World War, and Aberdeen during Alex Ferguson's reign - but those challenges have always subsided before too long. Against that backdrop, the sporting rivalry between the Glasgow clubs has been understandably fierce, but it is the religious divide that has taken it to another level.
The clubs grew up in opposition to one another, Celtic the team of the Irish Catholic immigrants, Rangers the team of the native Scottish Protestants. Celtic have historically been the more relaxed of the pair - after all, their greatest ever manager, Jock Stein, was a Protestant - as Rangers for a long time refused to allow any Catholic presence in their ranks. The signing of 14-year-old Catholic schoolboy John Spencer in 1985 saw some Gers fans refuse to return to Ibrox, while the response to the signing of former Celtic forward Mo Johnston in 1989, scarf- and season ticket-burning included, is well documented.
Superclásico (Boca Juniors vs River Plate)
Considered the most passionate of all football derbies, Boca and River are by some considerable distance the best-supported clubs in Argentina, and each edition of the Superclásico holds Buenos Aires in its grip for days.
The clubs began life in the working class docklands of La Boca, but River upped sticks to the more salubrious Núñez area in 1925, and the social divide helped fuel this most colourful and boisterous of rivalries. "I don't go anymore," one Boca fan told The Observer in 1997. "Every time you stop jumping or singing and take a rest, you get some idiot screaming in your ear: 'Sing! Sing!'"
Beyond the thrilling vibrancy, the derby has its dark side. In 1968, 71 fans were killed in a crush at River's El Monumental ground, while the clubs' barra brava - or ultras - have brought deliberate violence. In 1994, after River beat Boca 2-0 on their way to the Apertura title, Boca fans unleashed a hail of bullets on a lorry carrying River fans, killing two. Graffiti appeared in the aftermath reading 'River 2-2 Boca'.
Earlier this year, the Superclásico schedule was brought to a halt after River were relegated for the first time in their history.