The old port of St Tropez shimmers in a mosaic of purples, mauves, pale blues and golds as the sun sets in Paul Signac’s Impressionist vision of the Provencal fishing village. But the serenity of L’Annonciade museum – located on the very quayside where Signac painted his view back in 1899 – is suddenly dashed by loud bangs from somewhere in the real town.

More construction, maybe that multi-level car park they’re pile-driving in the back streets behind the still picturesque port? Whatever the noise is, it shatters the pleasure of art contemplation, so I exit – and find myself in the middle of a battle.

The explosions are nothing to do with the relentless development of the French Riviera’s hippest resort. Rather, they are the gunfire of a comic opera army parading along the harbour side in archaic uniforms, wielding muskets and blunderbusses.

Quite by chance, I’m witnessing the enactment of an explosive, centuries-old tradition by a goodly number of Tropezians – the permanent residents who number no more than 6,000 and become swamped by tourist hordes who can number 50,000 daily at the height of summer.

As this is early in the season, mid-May, the narrow streets are relatively untouristed, and a couple of hundred townsmen plus some girls and boys are performing in the annual Fête de la Bravade, a three-day bash. Lead by a fife and drum band, some dressed as soldiers with tall cockaded hats, the others as sailors, their flat hats topped with red pom-poms, they march for a couple of minutes along the quayside, then halt and stuff gunpowder down their gun barrels.

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They then shoot them downwards in an immense cacophony of gunfire and clouds of smoke, just like in the Napoleonic wars, except nobody loses any limbs. Aren’t these musketeers – some well into middle age – doing their ears an injury? They certainly are mine, and I take to sticking my fingers in my ears at every command of “Fire!” Then I notice all the performers are wearing earplugs.

As the parade passes legendary cafés and bars such as Sénéquier and the Bar du Port, their chic denizens have to do without the customary admiration of passers-by while all eyes are on the marching men in blue, white and red. The procession moves away from the harbour and back into the town, along the narrow Rue Allard, its seamless string of designer stores now emptied of customers. Who wants to be deafened while trying on those Gucci shades, so good for people-watching at the port, or that little black Prada dress that’s perfect for tonight at Les Caves du Roy, one of the world’s most outrageously expensive nightclubs?

For once, St Trop – as the French nick­name it – is not about fashion excess but historical respect. For a change, it’s not the visitors who are posing but the locals. But don’t ever say so.

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For Tropezians, the Bravade is deadly serious, a chance to honour their ancestors and the independence they fought hard to preserve – even if their lives are now totally dependent on the tourist trade. Nobody has a fishing boat nowadays, and the merchant fleet that used to ply the coast is long gone. That business made the town wealthy and funded the Genoese-style quayside houses from the 15th century, when St Tropez became a self-governing republic for 202 years.

To protect its tax-exempt prosperity, the republic set up its own forces to fight off pirates, Turks, Spaniards and other Mediterranean marauders, and it’s this militia – bearing the proud motto of Ad Usque Fidelis (“forever faithful”) – that is annually recreated for the Bravade. The meaning of “bravade” is roughly “act of defiance”, and the Tropezians turn out in costume on May 16 to 18 every year to raise a metaphorical finger to their enemies.

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St Trop isn’t merely the simple fishing village made famous by Brigitte Bardot’s sex appeal and turned into a jet-set resort. No, this is a place with a powerful history and inhabitants who are immensely proud of it.

The festival is organised by the Amis de la Bravade, a body to which most Tropezians belong and which is presided over by an official called the cépoun. The current cépoun, Serge Astézan, speaking to The Riviera Times, a local newspaper, made this crucial point: “The sincerity and belief of the participants is of primary importance. For us, the Bravade is not a festival but an important religious and military tradition. We don’t celebrate until afterwards.”

And that is why, no matter how opera buffa it may all seem to outsiders, the folk militia perform with utmost seriousness – and why wise visitors keep a straight face. Especially when Saint Torpès goes jogging by.

St Tropez got its name from Torpetius, an early Christian martyr whose corpse was dumped in a boat in Pisa and fetched up on this Provencal shore on May 17, AD68. To cut a long and implausible story short, the locals took him up as St Torpès, their patron saint, renamed their village after him and fashioned a twice life-size wooden bust of him to keep in the parish church.

Toted around in the Bravade, this figure forms the festival’s religious focus, carried on a litter by four dour men from Pisa in crimson medieval garb.

A most curious thing in the procession, this bulging-eyed bust with a huge diadem on his head trots by, attired in a crimson toga decked with heart-shaped lockets and St George crosses, escorted by antique warriors.

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Then they do it all again on June 15, in the Spaniards’ Bravade. This commemorates the great Tropezian victory of 1637, when 21 Spanish galleons attacked the town and were heroically driven back to sea. Again the figure of St Torpès is paraded through the streets, this time along with all the other saints’ figures of the parish church, the buildings tremble with fusillades from 100 muskets, and it all ends with a beautiful mass in the church.

St Tropez appears to be little more than a catwalk for the rich and beautiful; very pretty but totally vacuous. Yet behind this facade is a close-knit community rich in tradition and very proud of their history.

Just don’t laugh when they celebrate it.