Leftists who last year voted for Prime Minister Romano Prodi, an Iraq war opponent, turned out in droves to decry his approval for U.S. plans to expand the military base in the city of Vicenza, home to the 173rd Airborne Brigade.

Pacifists waved rainbow-striped peace banners while some protesters carried anti-American slogans like "Yankees go Home".

"There is no reason to have this base here," said Antonio Faitta, a 25-year-old gardener who traveled from Genoa.

Prodi appealed to demonstrators to refrain from violence, following warnings from the interior minister that the protest march which began shortly after 2 p.m. (1300 GMT) could attract people "hostile to the forces of law and order."

The U.S. embassy has warned Americans to steer clear of the small northern Italian city of 115,000, where officials also shut schools normally open on Saturday as a precaution.

The demonstration has also served as a lightning rod for anti-U.S. sentiment in a country where judges have ordered CIA agents and a U.S. soldier to stand trial for kidnapping and murder.

A Milan judge charged the CIA agents on Friday with abducting a Muslim cleric in Milan in a covert operation and flying him to Egypt. The U.S. soldier was charged on Feb. 7 with murdering an Italian secret agent in Iraq, although both governments have described the 2005 shooting as an accident.

All will almost certainly be tried in absentia, since Washington is not expected to hand them over.

"I don't want any more Americans here and I don't want a new base. They should just leave us alone," said Pucci Mori, a resident of Vicenza, who lives near the proposed base expansion.

"Wherever they go in the world, Americans cause trouble."

The protest is the latest headache for Prodi, who has faced revolts by his broad leftist coalition partners on everything from gay rights to the budget and the presence of Italian peacekeepers in Afghanistan.

The Pentagon wants to double the size of the base to unite its 173rd Airborne Brigade and expand its 2,750 military personnel to 4,500.

At present, the rapid reaction unit is divided among the base at Vicenza, about 400 km (250 miles) north of Rome, and bases at Bamburg and Schweinfurt in Germany.

The new barracks would be on the other side of the city from the existing one. That has raised worries about new roads to handle military traffic linking the two parts, loss of green space and strains on public services.

Residents fear it could even put Vicenza in danger.

"The people of Vicenza are concerned. The base would be in the heart of the city and in the case of a military conflict it could become a target," said Nobel literature laureate Dario Fo, who has organized a play about the U.S. base for later on Saturday.