Chelsea fans were allegedly attacked by Roma supporters ahead of their Champions League meeting on Tuesday.
The two sides will face off in the Italian capital a fortnight after a five-goal thriller at Stamford Bridge.
Roma fans clashed with police ahead of that encounter and there were more violent scenes on Monday night.
According to Corriere dello Sport , a group of around 40 travelling Chelsea fans were drinking at The Shamrock pub near the Coliseum when they were approached by locals holding metal bars.
Italian police soon arrived at the scene and arrested two Roma supporters.
A bystander told The Sun: "It all kicked off around midnight just as the pub was about to close up.
"This group of Roma fans burst in wearing masks and sticks and attacked the Chelsea fans who were heavily outnumbered.
"It was chaos for a few minutes and then when police arrived the Roma fans ran off but they stopped two of them."
No serious injuries are believed to have been suffered.
As many as 2,000 Chelsea fans are expected to be in attendance for the group stage fixture.
British and Italian police are working together to prevent further trouble before and after the encounter.
A number of past matches involving English sides in the Italian capital have been marred in recent years.
Last week Roma fans clashed with police in a swanky part of London ahead of the Champions League match in the capital.
Black-shirted supporters pushed and shoved Met officers before they regrouped and marched forward, causing the supporters to scatter.
Police had flocked to High Street, Kensington amid reports of ‘disorder’ with one eyewitness described ‘a bit of a melee’ before police moved on the crowd.
The Met Police confirmed one arrest had been made after fans clashed with uniformed police at 6pm. Around 10 police vans were sent to the scene, with a number of supporters said to be ‘throwing punches’.
The force later said they had made one arrest, while commuters talked of 'panic' as they left work and were greeted by the scenes.