FLORENCE, Italy, Nov. 9 — Hundreds of thousands of demonstrators marched through the streets here today to protest a possible military strike against Iraq, chanting antiwar slogans and throwing this Renaissance capital into a jittery state of alert.

The protesters represented a loose coalition of opponents of globalization who came here this week for a political conference. Tense Italian government officials feared a reprise of the bloodshed and chaos that erupted at an antiglobalization demonstration in Genoa last year.

About 5,000 police officers fanned out across the city to monitor the march and guard Florence's artistic and architectural treasures, some of which were also fenced off for protection. Hundreds of stores and restaurants closed, covering their glass facades with sheets of metal or plywood, as if preparing for a hurricane.

But by this evening, as marchers danced at a concert outside a stadium at the end of the four-mile route, there were no reports of serious violence.

The protesters toted placards, flags and banners in half a dozen European languages, many of which urged President Bush and Western European leaders not to attack Iraq.

"I think it's important to send a clear message to Bush and world leaders that if they go to war in Iraq, they're not doing it for a majority of people, and a lot of people object," said Darrell Goodliffe, 21, who had traveled to Florence from a small town near Cambridge, England.

Other demonstrators said their opposition to the war and their qualms over globalization were connected by a conviction that Western governments were motivated more by greed or imperialism and mistreating the world's less powerful people.

"We're protesting for peace in general, in every possible sense, in every possible meaning," Martina Cambi, 27, of Florence, said as she used eyeliner to paint Y-like shapes on the brows of friends. They worried aloud that the result looked more like Mercedes symbols than peace signs.

Amadeo Rossi, 48, of Turin, Italy, said he was demonstrating "against the war in Iraq, the mistreatment of immigrants and the abuses of the Italian government — all of the problems in the world."

Although forum organizers said there were as many as a million people at the march, the official government estimate was 450,000.

Many of the demonstrators arrived in Florence on Wednesday, at the beginning of a five-day conference of a coalition calling itself the European Social Forum. It was intended to unite various groups, from environmentalists and labor unions to latter-day Communists, with concerns about globalization.

The last huge antiglobalization demonstration in Italy was during a summit meeting of the world's major industrialized nations in Genoa in July 2001. Rioting broke out, and one protester was shot dead by a Carabinieri paramilitary officer, while hundreds more were wounded in clashes with the police.

Still haunted by that melee, Italian officials debated whether to allow demonstrators to gather here this week. They approved the event only after deciding to tighten border controls in an effort to turn away demonstrators with criminal backgrounds.

The event's organizers, for their part, agreed to move the route of the march, the highlight of the five-day gathering, away from the city center.

Even so, a fierce debate among Italians about the wisdom and merit of the forum persisted. On Wednesday, one of the country's leading newspapers, Corriere della Sera, published a front-page opinion piece by the journalist Oriana Fallaci, a native of Florence, who denounced the protesters and urged Florentines to spurn them.

"Don't even look at them," wrote Ms. Fallaci, who also recommended that Florentines shutter the entire city. She said the protesters were demanding peace from Mr. Bush, but not from President Saddam Hussein of Iraq or Osama bin Laden.

The days leading up to the march were peaceful, but many Florentines had already fled town, leaving the narrow cobblestone streets in the city center oddly deserted.

Demonstrators said Florentines had misunderstood their intentions. "There are no barbarians here, only young people against war who want to meet and exchange ideas," said Leonardo Sacchetti, a spokesman for the forum.

Those young people seemed to be in a frame of mind more festive than combative, and at one point, when a minor scuffle broke out between about a dozen protesters, other protesters shouted, "Shame! Shame!"

As the demonstrators marched, many blew whistles, a shrill sound that competed with music from a 25-piece band. Others ate pizza as they walked, while a few glided along on in-line skates. A young woman with face paint that resembled a clown's climbed up a tree, then swung around the branches as if they were uneven parallel bars.

Whenever demonstrators passed stores with boarded-up windows, they scribbled notes on the wood.

The message outside a closed McDonald's restaurant said, "We wouldn't have gone in, anyway."

On the plywood in front of a shop, someone had written, "Closed for stupidity." Someone else had scrawled, "Hello, Oriana."

But another marcher had left a slightly sinister message. "I will return when you're open, and then . . ." it said.