Clashes broke out inside Greece's Olympic Stadium ahead of the Champions League match between AEK Athens and Ajax.
Riot police dispersed home fans who threw plastic bottles and other objects at visiting supporters, and moments before the match started, riot police charged into the stands to break up AEK fans who hurled flares and smoke canisters.
No arrests or injuries were immediately reported, but images showed supporters in the stands left bloodied and an explosion going off before the match began.
Police also clashed with mostly Greek fans in parts of central Athens earlier Tuesday and late Monday. Rival Greek supporters were involved in the violence near the city's police headquarters. Traffic was blocked after youths hurled petrol bombs at rival supporters and police.
Ajax went on to win the match 2-0 to secure a spot in the round of 16, while hosts AEK are already eliminated from knockout round contention after collecting zero points from their five Champions League matches.