PARIS—Top clients of Le Bon Marché, the luxury department store, strode past velvet ropes in well-cut suits and cocktail dresses to attend a soiree with a special musical guest.

“We’re ‘Little Daylight’…from Brooklyn,” belted singer Nikki Taylor, before plunging into an electro-pop opus that made the store’s lacquered surfaces rumble. Nearby, rows of knitted beanie caps, artisanal jams and a hand-painted bicycle helmet were arrayed beneath ersatz rooftop water towers emblazoned with the slogan “Brooklyn Left Bank.”

Le Bon Marché—owned by luxury conglomerate LVMH Moët Hennessy Louis Vuitton SA and located on the rarefied Left Bank—was kicking off a six-week showcase of goods sourced from Brooklyn. But the department store was also staking a claim: The borough that birthed the contemporary hipster now belongs to Paris’s chic elite.

“Maybe it wasn’t obvious, but the real Brooklyn is now on the Left Bank,” says Jennifer Cuvillier, director of the store’s style office. “There is no more doubt.”

That salvo has ignited a Parisian battle for the soul of Brooklyn. For fashionistas, Le Bon Marché’s decision to plaster Paris in “Brooklyn’Mania” billboards portraying a tattooed hipster in a handlebar mustache is a fashion statement. Some longtime Brooklyn connoisseurs, however, see the event as a new low in the borough’s fall from gritty, countercultural grace.