Two Tottenham Hotspur fans have been stabbed in a night assault in a Rome pub by around 100 masked Lazio hooligans, Italian paper La Repubblica reported.
The two fans have been taken to hospital. One suffered injuries to an artery and is reportedly in a critical condition.
A total of ten supporters of the Tottenham team - nine British and one American - were hurt in the attack.
A police spokesman said five talians who were involved in the fight had been arrested.
Lazio fans have a notorious reputation, with a strong hooligan hardcore known as the Irriducibili.
They have long been associated with far right politics and a wide range of banners are often unveiled on the Curva Nord end of the Stadio Olimpico.
Banners showing support for Arkan, the Serbian war criminal and glorifying the Auschwitz concentration camps have been paraded before matches.
Paulo Di Canio, now a manager at Swindon, infamously celebrated a goal by running to the crowd and giving the Irriducibili a fascist salute after scoring during the Rome derby. The right hand raised in the air in a straight-armed Roman salute was a symbol of Italy's former fascist dictator Benito Mussolini, himself a big Lazio fan who often went to matches.