Violent clashes broke out inside Greece's Olympic Stadium ahead of a Champions League match between AEK Athens and Ajax on Tuesday night.
Home fans reportedly threw flares and a missile or petrol bomb which exploded close to the Ajax supporters.
Police then intervened, confronting the visiting fans in further violent scenes. No arrests or injuries were immediately reported, although photos showed several supporters with blood on their face following the clashes.
The incident followed incidents in parts of central Athens earlier on Tuesday, when rival Greek supporters were involved in the violence near the city's police headquarters. Traffic nearby came to a standstill after petrol bombs were hurled both at rival supporters and at police.
