PARIS — A threat to impose spot fines on women who sunbathe topless or in thongs on Paris Plage, a summer beach on the banks of the Seine, has left the city’s mayor struggling to maintain his carefully nurtured image as a modern civic chief.

In a country where going topless on real beaches is almost de rigueur, incredulity has greeted news that city hall officials and police have been moving among sunbathers, warning them of the ban on “indecent” dress.

What is especially embarrassing for Paris’s openly gay mayor, Bertrand Delanoe, is that Paris Plage, now in its fifth year, is intended to reproduce the ambience of an Atlantic or Mediterranean beach.

“Sand, sunbeds, parasols,” the tabloid daily Le Parisien said. “On the beach along the Seine, Bertrand Delanoe has deployed the full panoply of the perfect seaside postcard. But beware, it is a beach only in name and those who want sun-bronzed bottoms are unwelcome.”

The major newspaper proudly describes Paris Plage as an attraction not only for tourists but for Parisians too poor to join the summer exodus to the coast.

The riverside highway is closed to traffic and covered by 2,000 tons of fine sand. The beach stretches for more than two miles and is dotted with palm trees and cafes. But the order forbidding the exposure of flesh declares: “Behavior must conform to good morals, tranquility, safety, and public order.”

The penalty for going nude, topless, or in a thong is $48.