PARIS — Five armed, masked thieves sprinted into an art museum on the French Riviera during opening hours Sunday afternoon, snatched a Claude Monet and three other masterpieces off the walls, stuffed them in bags, and made their getaways by motorcycle and car, French authorities said yesterday.

The brazen robbers, clad in jumpsuits, ordered guards to lie on the floor at gunpoint as accomplices fanned through the galleries of Nice’s Museum of Fine Arts.

One thief grabbed French Impressionist Claude Monet’s evocative “The Cliff Near Dieppe” while his accomplices snared a painting of a poplar tree alley by impressionist Alfred Sisley, and two works by Flemish artist Jan Bruegel, according to the French Ministry of Culture.

The thieves attempted to steal a fifth painting, but it was too large to fit in their bags, according to witnesses cited in French press accounts. About half a dozen visitors were in the museum at the time, authorities said.

The five men then ran out of the ornate, ocher-colored 19th-century villa that houses the museum; two hopped on a motorcycle, and the three others leaped into a waiting car and sped away, authorities said. About half a dozen visitors were in the museum at the time, according to news reports.

“Who could expect to be held up in broad daylight like that?” Patricia Grimaud, deputy curator of the museum, said in a telephone interview from Nice yesterday. “They were really bold and quick, it took them only 10 minutes. I can’t find the right words to describe what they did — it was just an exceptional heist.”

Museum officials said the 1897 Monet and 1890 Sisley Impressionist paintings had been stolen in a previous incident involving the same museum nine years ago. The Monet was later found in a boat under repair at a marina and the Sisley was recovered in the sewers of Marseilles on France’s southern coast, according to French press reports.