Football in Greece is plagued by endemic fan violence, which often spreads to other sports, such as basketball and volleyball.
A Greek policeman was severely injured late on Thursday when a mob of volleyball fans set upon riot police outside an arena in Athens.
State television ERT said rioters shot a flare at a 30-year-old officer, which severed his thigh artery. The policeman is in a critical condition at a hospital.
The violence occurred during a volleyball match between rivals Olympiakos and Panathinaikos in the Greek capital.
Fighting outside the arena led to the game being abandoned.
ERT said about 150 people left the arena during the game to attack riot police stationed outside with flares, gasoline bombs and stones. They then returned to the venue.
Police made heavy use of tear gas, much of which wafted into the indoor sports ground.
Officers also prevented spectators from leaving the arena amid efforts to identify the person who fired the flare.
Security had been high for the game as the two clubs' supporters have a history of intense rivalry.
Greek football is plagued by endemic fan violence, which often spreads to other sports, such as basketball and volleyball.