The fixture - known as the derby of eternal enemies - has a long history of violent clashes between the two sets of supporters.
And as Olympiacos players warmed up ahead of the game at the Stadio Apostolos Nikolaidis, home fans invaded the playing area and attempted to get at them.
Police eventually were able to restore order, and the match kicked off around 15 minutes later than originally scheduled.
Panathinaikos went on to win 2-1 to move within three points of their arch rivals at the top of the Super League.
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