The Blues supporters were in the centre of Thessaloniki when a band of five locals turned violent.
According to reports in Greece, two Chelsea fans were attended to by an ambulance but did not want to go to the hospital.
The incident occurred in the Tsimiki Street area of Thessaloniki, one of the main drags in the Greek port city.
PAOK fans have a reputation around Europe for extremely passionate support, that has been known to become violent.
In local derbies — as well as their recent Champions League qualifier defeat against Benfica — the fans light thousands of flares to create a cauldron of fire atmosphere inside the Toumba Stadium.
Chelsea expect to be greeted with a charged atmosphere at the 29,000-seater stadium in the Greek port city.
Maurizio Sarri has left Eden Hazard at home for the trip, while Ruben Loftus-Cheek made the 1,300-mile journey.
Teen sensation Ethan Ampadu has also made the trip, after the club announced his new five-year contract.
The Welshman has yet to play for the Blues this term, but was in superb form for Ryan Giggs' side in their two Nations League games.
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