The 10, nine British and one American, were attacked outside a pub near the centre of Rome by dozens of masked men. One suffered injuries to an artery and is in a serious condition.
A local resident who witnessed the violence and called the police said the assailants appeared to have planned the attack.
"The Italians were very organised, with helmets, bats, even balaclavas. There were much fewer of the English," Giuseppe Tamborra said.
"I saw four people lying on the ground, one with his forehead cut open from here to here, probably with a stool taken from the bar."
The attack took place at the Drunken Ship, a popular tourist pub, which was left wrecked by the fight according to photographs published in Italian sports daily Gazzetta dello Sport which showed piles of smashed and overturned bar stools and a pool of blood on the cobblestones outside.
Commenting on their Facebook page the American owners of the bar, located in the central square of Campo dei Fiori, called the incident a "tragedy" and said they would consider claiming damages.
Italian newspaper La Repubblica described the scene as "urban warfare" and said up to 100 Lazio 'Ultras' - hardcore fans of the club who are often associated with far-right politics - had attacked the pub.
However, police said that among 15 Italians detained were two fans of Roma, Lazio's bitter rivals as the other Serie A team from the Italian capital, who were charged with violence and grievous bodily harm.
Lazio issued a statement saying any suggestion that the assailants were Lazio supporters was "totally groundless".
It said the episode was "pure delinquency" that had nothing to do with football and called for those responsible to be punished.
UEFA fined Lazio 40,000 euros (£32,000) last month for racist chanting directed at black players in a match against Tottenham in London in September, and Spurs manager Andre Villas Boas said he feared a repeat at Thursday's game in Rome.
Some Italian media reports said the attack may have been politically motivated, as sizeable sections of Lazio supporters express extreme right-wing sympathies, while many fans of the north London side claim a Jewish identity for the club, some referring to themselves as the "Yid Army".
Hundreds travelled over to see the Europa League match at the Stadio Olimpico.
Inside the stadium a group of Lazio supporters chanted "Juden Tottenham, Juden Tottenham" when the game got underway, and unfurled a banner saying "Free Palestine".
Tottenham fan Dave Illesly said he had left the bar shortly before the trouble started but feared further violence at the game later on.
"That is the trouble with football you never know, when you come abroad to foreign countries trouble comes to you," Illesly said. "I'm not really looking forward to it now, bit worried to be honest. I just don't know, I would rather go home."
"I'm going to cover up my Spurs shirt," fellow fan Sean Sill said.
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