Although the organizers were outwardly calm, the very size taken on by the demonstration made them "a bit frightened," central coordinator Salvatore Canavo admitted.

The organizers say Saturday's march will be a peaceful anti-war protest, despite fears by residents and government officials that it could turn violent.

The march against a possible US-led war on Iraq will mark the climax of the European Social Forum, a five-day gathering of the anti-globalization movement that has drawn 50,000 people, more than twice original estimates.

Saturday's march through the historic center of the Renaissance capital 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.

Canavo said he did not doubt that the march could result in disturbances but added that he hoped "that police will not turn to violence as they did in Genoa".

Saturday's march, scheduled to start at 1 pm (1200 GMT), was to cover a three-kilometer (1.8 mile) stretch from Florence's historic center to the Artemio Franchi football stadium, where an evening concert will close the demonstration.

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.

Although police will not close off the center of Tuscan city, some 6,000 security officers will be mobilized to protect the city's 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.