Eleven Manchester United fans were taken to hospital after violence erupted around their match against Roma, the British Embassy has said.
United fans clashed with baton-wielding police inside Rome's Stadio Olimpico.
Two of the fans were reportedly kept in hospital overnight. Two Roma supporters also had treatment.
Uefa is to investigate the trouble involving many fans from both sides. Italian police - whom some fans accused of brutality - justified their actions.
Local media also reported five people - three English and two from Italy - were injured in clashes as fans made their way to the stadium.
During the game, which Roma won 2-1, both sets of fans could be seen throwing missiles at each other.
The two sets of supporters were separated in one section of the ground by a barrier with riot police gathered on the Manchester United side.
After each goal both sets of fans surged towards the barrier.
At one stage, riot police with shields and batons were seen repeatedly striking Manchester United supporters.
Home Officer Minister Vernon Coaker told BBC Radio Five Live that answers were needed into whether or not Italian police had been brutal.
"The scenes that we saw on our television screens were extremely concerning," he said.
"And I would be interested to see the report that comes - not only from Italian police but also from our own police officers who were out there, from the Embassy to listen to the fans themselves and indeed from the club to see what their view of things is and we need some explanation of what actually took place and the police response to it."
Achille Serra, head of Rome Police, told BBC Rome's Christian Frazer the police action was a "justified response", and said there would be no inquiry unless he were shown evidence of any alleged police brutality.
Mr Serra added that if there were evidence of any brutality he would condemn it - but he said there was no such behaviour.
Stephen Fitzsimons, 53, from Stretford, Greater Manchester, was knifed in the groin as he made his way to the ground.
He told BBC Radio Manchester: "No police have been to see me... it's disgusting."
A Manchester United fan, David Pearce, said: "Italian police attacked the Manchester United crowd. I tried to help a girl who'd fallen down and one of the policemen hit me with a baton.
"It's absolute pandemonium. It's just terrible."
Another fan, Debbie Robertson, told the BBC she was shielded by Italian police from other officers who were using their batons to beat fans.
"I was very frightened. I was actually shielded by... one of the police came and shielded me from his colleagues."
One Rome resident, Justin Smith, e-mailed the BBC News website to say that before the game he had seen Manchester United fans "stumbling in and out of the pubs all day getting riotously drunk, to then show up at the stadium two hours before game time looking for trouble".
Around 5,000 United fans had tickets for the game.
William Gaillard, from Uefa, said the body was very concerned at the number of violent incidents this year.
"We will be waiting for the delegate's report, and the control and disciplinary body will be looking at the television images to see the dynamics of what actually happened."
Manchester United said in a statement: "There were clearly some very disturbing scenes in the stadium last night.
"Until we have had chance to discuss the situation with the British Embassy in Rome and the Italian authorities and to understand the cause, it would be inappropriate to comment."
Italian football has a reputation for violence among its fans.
Before the game Manchester United fans were warned in letters and on the club's website to avoid certain areas of the city, as they could be attacked by Roma's hardcore "ultra" fans.
The city's mayor Walter Veltroni criticised Manchester United officials and said the advice could create a "negative climate".
In February the Italian Football Federation (FIGC) suspended all matches after a policeman was killed at a Serie A game between Catania and Palermo.
Three Middlesbrough fans were stabbed and 10 others hurt during fights with Roma fans at an Uefa Cup quarter-final last year.