Tottenham Hotspur fans were set upon by about 50 masked thugs making Nazi salutes ahead of a football match in France.
The club confirmed three fans were hurt, and arrests have been made.
Up to 150 Spurs supporters were in the Smoking Dog pub in Lyon before their team's game in the French city on Thursday.
Witnesses said three Tottenham fans were taken to hospital, one with a head injury, and claimed the attack had an anti-Semitic motive.
The Foreign and Commonwealth Office (FCO) said it was aware of reports of an attack and was offering assistance.
Tottenham fans said the attackers, some making Nazi salutes, smashed doors and threw cast iron chairs, wooden objects, and a flare into the pub, where up to 150 supporters were drinking.
They said the attack on the bar, which is popular among ex-pats, happened at 10.18pm on Wednesday.
The thugs returned shortly afterwards to launch another assault on the pub, which was damaged extensively as a result.
A damaged door in the Smoking Dog pub after the attack
A Tottenham spokesman said: "Club officials attended an incident last night in Lyon. A group of around 50 people confronted fans.
"The situation was dealt with and arrests were made within the hour. Three fans sustained minor injuries and received prompt medical assistance.
"Spurs officials and stewards will be in the city throughout the day, along with matchday French police ahead of kick-off."
The English landlord, Dave Eales from Retford, Nottinghamshire, said: "At 10.18 the pub was rammed with Tottenham fans watching the Champions League on the television. The front of the pub, which is made up of reinforced glass, was attacked by 20-25 people.
"They threw projectiles through the windows. There was a stand off. They didn't get into the bar. Then it calmed down. Then they came back a second time and it all started again.
"There were three injured Tottenham fans taken away by ambulance. One of them had a head injury. I'm not sure how bad it was."
It is the second time that Tottenham fans have been targeted by an apparent anti-Semitic attack in the last three months.
In November, fans of the club, which has a traditionally strong Jewish following, were ambushed inside a pub in Rome on the eve of their game against Lazio.
Far-right thugs were blamed for that knife attack, and two witnesses claimed the assaults last night could have had an anti-Semitic motive.
One, who did not want to be named, said: "There were 50 who attacked in the first wave and 25 in the second.
"It's a pretty scary thing when you're confronted by people doing Nazi salutes.
"A lit flare was thrown through the window as well as the heavy weight, the kind that's used on terraces to hold down umbrellas.
"It was terrifying. I've never seen anything like it. They threw iron stools through the windows as well."
Spurs fan Christian Radnege, who was in the bar at the time of the attack, said: "They came in making Nazi salutes. It was frightening.
"Rome was fresh in everyone's minds. Everyone realised this was serious.
"They left and then we thought that was it, but five minutes later it started again.
"The intruders did not get in to the bar because it was full of Spurs fans. There were drips of blood left in the bar. The owners barricaded the pub between the two waves of attacks."
Tottenham fan James Taylor drove through the night from his home in Ilford, Essex, to attend tonight's Europa League second round second leg clash.
He had no doubt that this was not a random attack.
"It was pre-meditated, pre-arranged. It was nothing to do with Spurs fans," he said.
"We were in the pub having a few drinks at about 10pm. There were about 150 Spurs fans singing. It was all in good nature.
"Then there was a bang. We ignored the first one and there was a second one, suddenly. The windows were smashed and stools were thrown into the pub. We ran back to hide.
"We were stuck in the back of the pub for about half an hour. We were worried. No Spurs fans were violent or too drunk."
Eales, 42, bought the pub in 1995. He attended last week's first leg at White Hart Lane, which passed off without incident, as a Lyon fan, and had invited Spurs supporters to his bar while in London.
He says he had anticipated trouble could occur, and that he had told the Lyon police as much earlier this week.
He added: "The most disappointing thing is that I spoke to the police a couple of days ago about the potential for trouble because of what people associate Tottenham being.
"There will be a big presence on the match day, but tonight it didn't happen."
A spokesman for the FCO said: "We are aware of reports of an attack on British nationals in France and are offering consular assistance."
Tottenham Hotspur Football Club said they were aware of reports of the attack.