Tens of thousands of people are taking part in a mass rally in the northern Italian town of Vicenza against the expansion of a local US military base.
Police estimate the number of demonstrators at 25,000, but reports from the scene suggest it could be more than 100,000.
The arrest of 15 suspected Red Brigade terrorists earlier this week prompted fears that extremists could stoke violence at the protest.
Ahead of the rally, the Italian Prime Minister, Romano Prodi, appealed for demonstrators not to use violence. About 1,500 police and gendarmes are on duty to prevent trouble.
The US embassy put out a travel advisory urging US nationals to stay away for the last two days.
However, protestors have insisted their demonstration is a peaceful one.
A former mayor of Vicenza, Achille Variati, said the protests were also being misconstrued as anti-American.
'It's about us against the city administration,' he said. 'It would be a big error to be anti-American.'
Move would double US presence: ex-mayor
Washington wants to centralize the US 173rd Airborne Brigade, which is currently spread across two sites in Germany and at Camp Ederle, on the east side of Vicenza.
Under the plan, the 2,750-strong Brigade will consolidate in Italy by taking in a former military airfield at Dal Molin on the opposite side of Vicenza at a cost of $500 million.
The plan would more than double the US military footprint in the town, Mr Variati said.
Local and regional authorities are in favor of the plan, but it is fiercely opposed by pacifists, environmentalists and residents, despite the some 1,200 local jobs provided by the base, one of seven US bases in Italy.
The expansion is said to be an embarrassment for the centre-left government, which has backed the plans even though left wing members of Mr Prodi's administration oppose it.
