PARIS, JULY 5 -- Officials permanently closed five Paris museums to individual visitors today, an unprecedented step prompted by three art thefts in one day, including a Renoir sliced from its frame at the Louvre.

The city's major museums will remain open, but security will be tightened, said Jacques Sallois, director of national museums. Authorities plan to examine existing security systems and develop new ones similar to those used in banks.

Police said it was possible that the same person or group hit all three museums during a span of several hours Wednesday, moving from one museum to another in the middle of the day.

In addition to the Renoir, an 1816 painting by Paul Huet was stolen from the Carnavalet Museum, and an 1870 work by Ernest Hebert was stolen from a museum named after the artist.

The Hebert was one of the five museums closed to individual visitors. The others -- also relatively small institutions focusing on the works or the collections of a particular individual -- are the Eugene Delacroix, Gustave Moreau, Jean-Jacques Henner and Ennery museums.

From now on, group visits to these museums must be arranged in advance, Sallois said.

Police have complained previously that museum officials were not aggressive enough or candid enough in dealing with security. Sallois, who took up his post only five weeks ago, vowed this would change, and said art theft would no longer be a "taboo subject" for public discussion.

The Louvre's director, Michel Laclotte, said the theft of Renoir's "Portrait of a Seated Woman" appeared to be the work of an "especially clever" professional.

"The canvas was very thin; the cutting instrument was probably extremely sharp and caused so few vibrations that the electronic sensor did not pick them up," he said.

Laclotte led reporters to the spot where the portrait had been hanging -- an isolated stairwell leading to the Denon Gallery, on the top floor of the Pavillon de Flore overlooking the Seine.

Head painting curator Pierre Rosenberg said the Renoir had been on display in the stairwell for several years, because of renovations in other sections of the museum, once a royal palace.

Investigators said the theft from the Carnavalet Museum apparently was the first to occur. The work by Huet, a French landscape painter, was lifted off its hook Wednesday morning at the history museum in one of the oldest sections of Paris.

The painting, "Les Moulins de la Glacie`re," was estimated to be worth about $14,500.

Officials gave no estimates of the value of the Renoir or Hebert works. Rosenberg said he thought estimates of $300,000 to $600,000 for the Renoir, given by independent experts, were high.

The Renoir and Hebert's "Portrait of Monaluccia" both were cut from their frames. They are small paintings, a type often chosen by thieves because they can be easily concealed.

"Portrait of a Seated Woman" measures 13 1/2 inches by 10 1/2 inches, and Hebert's work is only slightly larger.