Four people were arrested and three injured following a clash outside a Panathinaikos supporters? club in Petralona, southern Athens late on Monday.
About 100 Olympiakos fans, some armed with knives and other weapons, attacked the fan club.
Two of the three people injured remained in the hospital yesterday.
The four people arrested are in their 20s and 30s.
Police said that they are seeking more suspects.
It was not clear what triggered the attack, but relations between the supporters of the rival teams have always been antagonistic.