Demonstrators clashed briefly with Italian police in the first big protest ahead of this week's G-8 summit of the world's richest nations.
Police in riot gear fired tear gas at protesters to prevent them from crossing a bridge and moving closer to a U.S. military base in the northeastern city of Vicenza.
A group of demonstrators, some wearing motorcycle helmets and with their faces covered, threw bottles, rocks and firecrackers as they were pushed back by security forces.
Protesters were later allowed to continue their march, which broke up peacefully in the early evening. There were no reports of injuries.
Several thousand people attended the demonstration, launched against expansion plans that would make the U.S. base one of the biggest in Europe, and more generally against the economic summit, Italy will chair Wednesday through Friday.