on the eastern side of the stadium, bearing the names of all 21 supporters killed on that day in the stadium. Even though this incident affected almost solely the fanbase of Olympiacos, other teams occasionally pay their respects to the people killed as well, as they consider the incident to be a tragedy not only for one team, but for the whole country. In the past, even foreign teams, such as Liverpool and Red Star Belgrade, have honoured the incident's victims. Rivalries Traditionally, Olympiacos' main rival is Panathinaikos and their so-called "derby of the eternal enemies" is a classic local derby in Attica, the most famous fixture in Greek football and one of the most well known around the world. The two clubs are the most successful, having won together a total of 67 League titles (Olympiacos 47, Panathinaikos 20), and the most popular football clubs in Greece. The rivalry also encompasses social, cultural and regional differences; Olympiacos, coming from the famous port of Piraeus, used to be very popular in the working to middle classes, while Panathinaikos, of downtown Athens, was considered the representative of middle to higher social classes, although this differentiation has weakened nowadays and the two clubs have similar fanbases. Most recent notorious incidents include a fan's death in 2007, during a pre-arranged clash between hooligans on the occasion of a women's volleyball game between the two clubs, which caused major upset in Greece, and the abandonment of a derby in 2012 after riots at the Athens Olympic Stadium, which resulted in major fires in parts of it. Olympiacos also shares a traditional rivalry with AEK Athens, in one more local derby of the Greek capital with the other member of the so-called Big three, but also with PAOK, in the fiercest inter-city rivalry in Greece between the most popular clubs of the two largest Greek cities, Athens and Thessaloniki, a rivalry that erupted in the 1960s for the sake of footballer Giorgos Koudas. A popular rivalry used to be the Piraeus derby, between Olympiacos and Ethnikos Piraeus, the second most successful football club in the region, but the fixture has faded-out due to Ethnikos' constant presence in lower divisions in the last decades. It remains a derby in water polo where Olympiacos and Ethnikos compete in the top division. European performance Olympiacos has a long presence in the UEFA competitions, debuting on 13 September 1959,