It was a strangely beautiful sight: between two and three hundred Italian riot police in immaculate blue uniforms, formed into a classic (and classical) Roman phalanx, and occupying every last square millimetre of the Accademia bridge in Venice. The sky was blue, the water of the Grand Canal was blue, the boys with the plastic shields, tear gas guns and three-foot truncheons wore blue.

It was clearly a beautiful sight, for mildly bewildered tourists trying to cross the timber bridge last Sunday saw this immaculate formation of urban commandos as a great photo-opportunity. Was it some sort of fancy- dress parade, part of a new September carnivale? As modern cameras wheezed and flashed, the ragazzi in blue stood impassively behind an upright young officer talking purposefully into a radio-mike.

The unusual scene was animated by the basso-profundo flapping of helicopters hovering over St Mark's Square and the stroboscopic flash of blue lights as energetic police boats raced along streets full of water.

"This is just great," said an American student from under a vast baseball cap. "Sure," said his buddy in Day-Glo leisurewear, perched on high-rise trainers, "but I kinda wonder what's going on?"

As they spoke, the police phalanx appeared twitchy, as if it might move very fast in our direction at any moment. At which point, a Japanese tour guide - one of those imperious ladies with a piercing voice and an umbrella brandished high like some medieval banner - led a troop of Japanese tourists from out of the Accademia gallery and marched them up to the bridge. Clearly her platoon was working to a very hurried schedule: Accademia (Bellini, Carpaccio, that sort of thing) dispatched in 20 minutes; next stop St Mark's famous square with 10 minutes for videoing the pigeons, before a forced march through the Doge's Palace and a photo-opportunity at the Bridge of Sighs.

Onwards they marched, these impervious Japanese, eyes glued unwaveringly to the latest generation of video-cameras. Theirs not to reason why, theirs but to make photography, into the Accademia phalanx they strode.

Forced to a halt by the police troop, the gallant tour guide tried to force her way through. She did not address the officer in charge, nor did she seem to see the armed ragazzi: she had a timetable to stick to and Italian people, police included, must not get in the way. This unseemly blockade, she feared, might delay the embarkation of the post-pasta gondola flotilla.

As for her charges, they videoed to the left of them, they videoed to the right of them, refusing point-blank to look ahead into that solid wall of military blue. It was as if the riot police did not exist. Unable to breach this ineffable wall of muscle and shields, the tour guide finally marched her band to the boat stop: now those statutory 10 minutes in St Mark's would have to be cut in two.

The Japanese seemed unable to acknowledge the presence of riot police in Venice last Sunday, whilst other tourists I spoke to were mystified by this unlikely phenomenon. Yet, at the bottom of the steps of the Accademia bridge is a news-stand, and pasted to the front and sides of that news- stand, in full view of tourists from all points occidental and oriental, were posters calling attention to the invasion of Venice by the green- shirted brigades of the appropriately named Signor Bossi. Bossi is the right-wing separatist whose aim is to create a breakaway northern Italian state, which he calls Padania.

A fight did break out in the port of Chioggia, at the far end of the Venetian lagoon, between riot police and a 40,000-strong band of flag- waving padanianisti; in Venice the threat was effectively contained by the riot police.

The truly disturbing thing was this: very few of the day trippers and holiday-makers and none of the video-crazy Japanese tourists appeared to have any idea that at any moment they might be caught up in a riot, in a fog of CS gas or even a hail of rubber bullets. Venice to them is a kind of crumbling and slightly smelly Disneyland, very historic, very amusing. But not a real place where political feelings can, and do, run as high as the campanile of St Mark's.

Holidays are a time for escape, yet it seems a wise idea for tourists to read the local newspapers, or at least to talk to local people and to ask for the latest state of play. Last Sunday, it was all too easy to imagine a group of tourists on a trip to the Bay of Naples on that fateful day in AD79 when Vesuvius chose to erupt, videoing whilst Pompeii burned. And if tourists are unable to read newspapers, perhaps tour companies should encourage their guides to do so. Riot police seen through the lens of the latest satellite-linked/digital/web-site/CD-ROM video camera may look strangely beautiful, especially when occupying Accademia bridge, but violence is only ever ugly.

Jonathan Glancey