Fabrice Pierret is used to catering to hordes of tourists who pack Le Lutétia, a brasserie he manages on the Île Saint-Louis, where a splendid view of the Seine River, with a glass of red wine and steak frites, has long made it a popular stop for visitors.

But with the opening ceremony of the 2024 Paris Olympics just days away, the crowds have thinned to a trickle. Business is down 50 percent — and more for shopkeepers nearby — as tough new security measures and an unexpected drop in tourism turns some of Paris’s most famous venues into veritable ghost towns.

“It’s a catastrophe,” said Mr. Pierret, surveying his near-empty terrace. Before him, thousands of brightly colored bleacher seats lined the quays of the Seine, which are now cordoned off. The river flowed by quietly — devoid of traffic — a scene reminiscent of Covid lockdowns.

“The Olympics were supposed to be great for business,” Mr. Pierret said. “Instead, we’re being hit really hard.”

Businesses have been counting on the Olympics to bring an economic boom. The city is turning into a giant outdoor sports venue, starting with the glittering opening ceremony Friday, when a flotilla will ferry athletes on a four-mile stretch of the Seine to the Eiffel Tower, with more than 300,000 spectators lining the route.