TOMORROW at noon, a rocket launched from the balcony of Pamplona town hall will signal the start of a round-the-clock fiesta of drink and danger, the highlight of which is the daily running of bulls through the town's narrow streets.

At the signal, white-clad revellers jamming the main square will erupt with joy, knot scarlet handkerchiefs around their necks and start pelting each other with eggs, flour, saffron and fizzy wine. The symbolic gesture honours San Fermin, an eighth-century martyr beheaded for preaching Christianity in a Muslim land. Those crammed into surrounding balconies draw back, drenched, shrieking and stinking.

This conservative northern Spanish town, renowned 51 weeks of the year for its sober prosperity and straight-laced rectitude, is sensationally transformed for a week starting on July 6, when it hosts some 500,000 enthusiasts from across the globe.

Preparations were in place at the weekend. Scarlet banners adorned the main square, portaloos were everywhere, fairground stalls lined the handsome main boulevard and drum and pipe bands noisily limbered up for the roaring week ahead. Legions of backpackers rolled into town.

"The atmosphere is euphoric," says Briton John Green who heads for Pamplona today for his 40th visit.

"It's very good fun and the consumption of alcohol is enormous. People seem to reach a stage of intoxication where they are still amusing, or they pass out. There's no drunken fighting and the number of incidents caused by drunken behaviour is minimal."

Naked "blood-spattered" animal-rights activists mount a town-centre protest against bullfighting at noon today. Their worldwide campaign has helped increase opposition among Spaniards to the practice. But far from spoiling the fun, Pamplona's annual protest has become part of the fiesta. Enthusiasts greet the protesters with claps and cheers.

Tomorrow night, garbage trucks will rumble through town crunching some 25 tons of shattered bottles, hosing alcohol from the cobblestones - and that's before the bulls have run.

On Tuesday, at 8am sharp, six fighting bulls will be released from their overnight corral on the edge of town and driven half a mile through barricaded streets to the bullring, where they face death in the afternoon.

The tradition began when herdsmen drove bulls to the ring early in the morning for the fiesta's daily bullfight. Locals joined in, keeping abreast of the beasts, testing their bravery as they dodged the deadly horns and hooves. Their bravado, hailed by Ernest Hemingway in The Sun Also Rises, became today's international spectacle.

The sprint, of two to five minutes, is broadcast live every morning with a commentary as expert and breathless as for the Grand National.

Police clear the route 30 minutes before the bulls are set loose, ejecting anyone who looks drunk. Dramatic pile-ups occur as men and beasts skid and tumble on the ancient cobbles. The hyped-up runners, who rarely last the full stretch, pile into cafes afterwards and dissect every swerve and spill while their excitement subsides.

Most casualties occur during the run, or from falling over drunk: around a dozen injuries annually, 14 deaths in 100 years. The last bullrunning death was in 2003 when a bull trampled a local man, Fermin Etxberri, 63, in the head. The last fatal goring, of the young American Matthew Tassio, was in 1995. He died because he did not know bullrunning's cardinal rule: "when you fall, stay down." Last year an Irishman fell to his death from the city ramparts.

"I'm not a psychologist, but on paper this festival is a disaster," says a senior police officer and old Pamplona hand. "Hundreds of thousands of people who don't speak each other's languages drink themselves to oblivion. But it's just one big party. Why? I don't know."

Equally paradoxical is the way the usually formal Pamplona explodes into uncontrolled, sometimes extreme, behaviour. "The unspoken rule is that you are allowed to do absolutely anything as long as you don't restrict the enjoyment of anyone else," says Green. "You can be as silly as you like, without hurting anyone. You absorb a self-discipline which means you throw up round the corner, rather than in someone's lap."

It's the embodiment of a libertarian ideal to which any anarchist might aspire: this tolerant philosophy flourishes only at fiesta time and permits a bizarre sideshow that has nothing to do with bulls.

Pamplona has become a rendezvous for young Australians and New Zealanders who, mostly in a drunken haze, scale a pillar in an old part of town, then hurl themselves into what they hope are the protective arms of their friends. Authorities banned the practice some years back, then lifted the ban, arguing that those risking their lives were not harming anyone else.

The free-thinking principle may be problematic as a theory of government, but it works well for a week of high-octane enjoyment.