Greek fans have a rep for being close to the most brutal.
Yes,some manage to bring in knives and other weapons.One year ago a fan of Panathinaikos was killed by an arranged clash through internet.
My brother who studied in Italy told me that once PAOK fans from Thessaloniki went with their team to a city in Sicely which is mafia center and after the match they proceeded to vandalisms while whole Italy looked astonished since noone dares to do anything in this city.The locals were caught by surprise.