A group of demonstrators, some of them wearing motorcycle helmets and with their faces covered, threw bottles, rocks and lit firecrackers before 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 US base one of the biggest in Europe, and more generally against the July 8-10 G8 summit, which Italy chairs.
"We are sick of the powerful governing without consulting the people," said Martina Vultaggio, 29, one of the protest organisers.
The leaders of the United States, Japan, Germany, France, Britain, Italy, Canada and Russia, together with those of major emerging economies, will hold talks in the central city of L'Aquila focusing on the state of the world economy, financial regulation, climate change, trade and development.
More protests planned
Anti-capitalist protesters have planned a series of demonstrations at different sites, starting with the one in Vicenza, where locals oppose the doubling of the size of the US base, home to 3,000 soldiers of the 173 Airborne Brigade.
Opponents contend the base poses a threat to ground water, is dangerous for residents and for Vicenza's historic centre, a treasure of Renaissance architecture.
The protesters had vowed to march about 3.5km to the construction site, which was sealed off by local authorities, and plant flags with anti-base slogans.
Around 1,000 police were deployed on Saturday along the route.
Violence marred a G8 summit hosted by Italy in Genoa in 2001, when a protester was killed and scores of others were beaten up by police.