FLORENCE, Italy, November 9 (News Agencies) - Hundreds of thousands of demonstrators marched through Florence Saturday to oppose a war on Iraq in what could prove to be the world's biggest street protest yet against U.S. sabre-rattling toward Baghdad.

Between 400,000 and one million people -- according to respective police and organizers' estimates -- braved the cold to march through this heavily policed Renaissance capital, but the mood was festive with no reported incidents, Agence France-Presse (AFP) reported.

Organizers had promised a peaceful rally though officials and locals feared it could turn into a violent repeat of last year's G-8 summit riots in Genoa and many shops were closed and shuttered.

The rally had been called to mark the climax of the European Social Forum, a five-day gathering of the anti-globalization movement that drew 50,000 people, more than twice original estimates.

But with events on Iraq moving quickly, the war theme took precedence. The protest came a day after the UN Security Council passed by unanimity a U.S.-proposed resolution which required Iraqi leader Saddam Hussein to abolish Iraq's alleged weapons of mass destruction.

The mostly Italian demonstrators -- anti-globalists, union activists, pacifists -- marched behind banners proclaiming "No to War" as they wove their way along a three-kilometer (1.8 mile) stretch that skirted Florence's historic center and ended up at the city's football stadium. An evening concert was to close events for the day.

Although police have not closed off the center of this Tuscan capital, more than 4,000 officers were mobilized to protect its treasures and monuments -- including Michelangelo's famous sculpture David -- many dating back to the Renaissance. Officials have also prepared a hundred cells in a nearby prison.

The city's famed museums and central cathedral, Il Duomo, with its landmark 15th-century dome by Brunelleschi were also kept open, but many shops had boarded up tight.

The march through Florence has come under heavy criticism from Italy's center-right government and conservatives, who fear a repeat of the violence at last year's Group of Eight summit in Genoa, in which a demonstrator was killed and hundreds were injured. About 300,000 demonstrators attended the Genoa summit.

The CGIL -- Italy's largest trade union and the group charged with the delicate task of maintaining order at the march -- has said the march will not end like the Genoa demonstrations.

The head of the EU executive, the European Commission, Italian Romano Prodi, said he would watch and see what happens Saturday.

"I will be watching what happens in Florence," he said at a congress in the northern city of Bologna. "The young people must be heard, which does not mean their opinions must always be shared."

"But in this time of change, the voice of youth is of major importance," Prodi said.

The anti-globalization movement, which includes a vast range of groups from environmentalists and pacifists to anarchists, argues that governments and international institutions like the International Monetary Fund put the interests of big companies ahead of those of ordinary people.

Protests were staged in several capitals at the end of September against the Bush administration's stance on Iraq. The largest of these -- in London and Rome -- drew about 100,000 people each.

Last year, One male protester was killed, one young woman demonstrator seriously injured and a police officer "very seriously injured" during clashes outside the G8 summit in Genoa last year.

Witnesses in Genoa told AFP that a young man had been shot in the head in the Via Caffa, a street leading to the summit venue.