Demonstrators threw stones, smoke grenades and firecrackers at riot police after baton-wielding officers fought them back when they tried to break through a security cordon to stage an unauthorised march near the French embassy.
About 600 protesters were involved in the clashes, which broke out when a group of helmet-wearing activists, their faces covered, tried to break the police cordon around the Campo de' Fiori square in the Italian capital, where authorities had told the demonstrators to remain.
"It's a useless project that won't serve any purpose except to spend public money," protest leader Paolo Prieri said of the plan to build a high-speed rail link between Lyon in eastern France and Turin, about 300 kilometres (190 miles) away in northwestern Italy.
French President Francois Hollande and Italian Prime Minister Enrico Letta were expected to discuss the project during their summit.
Launched in 2001, the rail link is meant to halve travel times between Lyon and Turin and cut the Paris-Milan journey from seven hours to four, while taking a million trucks off the road and averting three million tonnes of greenhouse gas emissions per year.
But the €26 billion project has faced fierce, sometimes violent opposition in Italy from environmentalists and affected residents who say it is wasteful and unnecessary.
It is also running behind schedule, its start date pushed back to 2025 at the earliest.