The US 173rd Airborne Brigade is currently based at two sites in Germany and Camp Ederle, on the east side of Vicenza, and Washington wants to consolidate the brigade in Vicenza.
|"Demonstrations are the salt of democracy, but they should be peaceful, serene and without violence"|
Romano Prodi, Italy's prime minister
The US embassy has put out a travel advisory urging US nationals to stay away from Vicenza on Friday and Saturday.
The Italian press predicted a turnout of up to 80,000, with the authorities warning of violence fomented by "extremists," while organisers scoff at the idea.
Oscar Mancini, the Vicenza regional representative for Italy's largest labour union, the CGIL, said at a pre-march rally on Friday: "The mass media are calling us extremists. They haven't understood anything about Vicenza."
In December, he recalled, more than 20,000 people held a peaceful protest in Vicenza against the base expansion.
Prodi on Saturday urged a peaceful protest, saying: "Demonstrations are the salt of democracy, but they should be peaceful, serene and without violence."
Achille Variati, Vicenza's former mayor said the protests were also misconstrued as anti-American.
He said: "It's about us against the city administration. It would be a big error to be anti-American."
Ronald Spogli, the US Ambassador to Rome, was hounded by protesters during a visit to Vicenza, which is some 40k west of Venice.
The base issue is one of several that are troubling US-Italian relations since Prodi came to power in May and withdrew Italy's troops from Iraq in December.