Paris - A broken alarm system made it as easy as 1-2-3: A masked intruder clipped a padlock, smashed a window and stole a Picasso, a Matisse and three other masterpieces from a Paris museum Thursday - a $123 million haul that is one of the world's biggest art heists.

Offloading the artwork may prove a tougher task, however, with Interpol and collectors worldwide now on high alert.

In what seemed like an art thief's fantasy, the alarm system had been broken since March in parts of the Paris Museum of Modern Art, according to the city's mayor, Bertrand Delanoe.

The museum, in a tony neighborhood across the Seine River from the Eiffel Tower, reopened in 2006 after spending $18 million and two years upgrading its security system. Spare parts had been ordered to fix the alarm but had not yet arrived, the mayor said in a statement.

So with no alarm to worry about, a lone masked intruder entered the museum about 3:50 a.m., said Christophe Girard, deputy culture secretary at Paris City Hall. The thief cut a padlock on a gate, then broke a side window and climbed inside - his movements caught on one of the museum's functioning cameras, according to the Paris prosecutor's office.

The intruder later slipped back out, carrying the canvases and leaving behind empty frames. The whole thing took 15 minutes, a police official said.

Three security guards were on duty overnight, but "they saw nothing," Girard said. A night watchman discovered the theft around 7 a.m.

The stolen works included Picasso's "Le pigeon aux petits-pois" (The Pigeon with the Peas), an ochre-toned Cubist oil painting worth an estimated $28 million, and La Pastorale" (Pastoral), a pastel-hued oil painting of nudes on a hillside by Henri Matisse worth about $17.5 million, Girard said.

Also seized were "La femme a l'eventail" (Woman with a Fan) by Amedeo Modigliani, "L'olivier pres de l'Estaque" (Olive Tree near Estaque) by Georges Braque and "Nature morte aux chandeliers" (Still Life with Chandeliers) by Fernand Leger.

Estimates of the total value of the paintings varied: The prosecutor's office initially put their worth as high as $613 million but later downgraded the figure to about $111 million. Girard said the total value was about $123 million.

The broken alarm system also renewed concerns about museum security in the French capital. There was no operating surveillance system when a thief made off with a red sketchbook of 33 Picasso drawings from the nearby Picasso Museum while it was undergoing renovations last summer.

Within hours of Thursday's heist, red-and-white tape surrounded the Museum of Modern Art and signs on the Art Deco building's ornate bronze doors said it was closed for "technical reasons."

On a cordoned-off balcony, police wearing blue gloves and face masks examined the museum's broken window and the discarded frames. The paintings appeared to have been carefully removed from the disassembled frames, not sliced out.

Investigators were trying to determine whether the intruder was operating alone, Girard told reporters. Stephane Thefo, a specialist at Interpol who handles international art theft investigations, expressed doubt that one person could have pulled it off the heist, even if only one person was caught on camera.