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VICENZA, Italy, July 4 (Reuters) - Anti-G8 demonstrators clashed briefly with Italian police on Saturday in the first big protest ahead of next week's summit of the world's richest nations.

Police in riot gear fired teargas at protesters to prevent them from crossing a bridge and moving closer to a controversial U.S. military base in the northeastern city of Vicenza.

A group of demonstrators, some of them wearing motorcycle helmets and with their faces covered, threw bottles, rocks and lit firecrackers 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 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.

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 U.S. base, home to 3,000 soldiers of the 173 Airborne Brigade.

The Italian government has approved construction of a new 6,000 square metre (64,600 sq ft) base on the site of the old Molin airport on the city's outskirts. But Vicenza residents have rejected the base expansion in a referendum.

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 3.5 km (2 miles) 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.

Prime Minister Silvio Berlusconi is keen to avoid a repeat of the violence that marred a 2001 G8 summit hosted by Italy in Genoa, when a protester was killed and scores of others were beaten up by police.

Summit organisers have said the choice of L'Aquila, which was badly damaged by an earthquake in April that killed nearly 300 people and left 60,000 homeless, should deter violent protests out of respect for the plight of local people.

Demonstrators plan to stage a candle-lit night march in the city on July 6 -- three months after the quake struck. (Writing by Silvia Aloisi; Editing by Sophie Hares)