Two Liverpool fans suffered head injuries after they were ambushed by Napoli ultras on scooters and hit with bottles and belts.
The Reds will take on the Italian side in the Champions League clash tonight as they begin their defence of the trophy.
The fans were in a group having a drink at a bar in the southern Italian city when they were attacked.
Liverpool supporters were advised by the club and the Foreign Office to avoid wearing club colours ahead of the game.
One fan told the Liverpool Echo: “We were just trying to have a quiet drink before going to the apartment and had just paid our bar bill. Next minute we could see about 10 scooters flying down. They slowed right down and their faces were covered with scarves and helmets.
“Next second they are jumping off swinging belts towards us. Must have all happened in 90 seconds but some lad got hit by a belt.
“Twice I’ve had bother here now. I am never coming back.”
Merseyside Police confirmed the attack.
A spokesperson said: “We can confirm two Liverpool fans were reported to have been assaulted in Naples earlier today (Tuesday 17 September).
“At around 6pm local time (5pm GMT), it was reported a number of males on scooters approached the fans at a bar in the city before assaulting them.
“Two men from the UK, aged 26 and 46, sustained superficial cuts and grazes to the head and were treated at the scene before continuing to attend the match.
“Local police area aware and Merseyside Police are working with police colleagues in Naples to assist in the policing of the Champions League fixture tonight (Tuesday).”