Hardcore Italian football fans known as "Ultras" hunted down and attacked Liverpool supporters in Naples 24 hours before the team were due to play in the Uefa Europa League, police said.
There were several incidents on Wednesday night ahead of last night's tie between Napoli and Liverpool at the city's Stadio San Paolo, despite police stepping up security ahead of the game.
Three Liverpool fans were treated in hospital for stab wounds, while a father and his two sons and their friend were also injured. One of the victims had been released from hospital last night, while two others, 53-year-old Alexander Phillips and a 27-year-old, were due to be released later.
Mr Phillips had injuries to his buttocks and thighs, and the other man was wounded in the head. The other four were injured after being attacked by a group of 20 to 30 men.
Violent "Ultra" followers of many Italian football teams have a reputation for stabbing rival fans in the buttocks, aimed to cause pain and humiliation but not fatal injury.
Three people were arrested over the attacks, Italian police confirmed.