French police are hunting the burglar or burglars who made off with paintings by Picasso, Matisse and other great artists from a Paris museum.
Officials have admitted the Museum of Modern Art's alarm system had not been fully functioning for several weeks.
One masked intruder was spotted by security cameras, climbing into the museum through a broken side window, having cut through a gate padlock.
The paintings are estimated to be worth just under 100m euros (£86m; $123m).
But experts say the thieves would struggle to sell the paintings on the open market because they are so well known.
The five missing paintings are Dove with Green Peas by Pablo Picasso (painted in 1911), Pastoral by Henri Matisse (1906), Olive Tree near l'Estaque by Georges Braque (1906), Woman with Fan by Amedeo Modigliani (1919) and Still Life with Candlestick by Fernand Leger (1922).
Paris Mayor Bertrand Delanoe admitted one of the museum's alarms had been "partly malfunctioning" since the end of March, and had been awaiting repair when the theft happened.
He called the incident "an intolerable attack on the universal cultural heritage in Paris".
Officials said that the burglar entered the museum at just before 0400 local time (0200 GMT) on Thursday, and took just 15 minutes to remove the canvases from their frames and slip out of the building again undetected.
Three security guards were on duty during the night, but the theft was only discovered at around 0700 local time (0500 GMT), Christophe Girard, deputy culture secretary at Paris City Hall said.
Mr Girard added that investigators were trying to determine whether the theft was carried out by one burglar or a gang.
"This looks like an operation by a professional gang, by organised criminals," he said. "We are dealing with an extremely high level of sophistication.
"To get into the museum by disassembling a window, choose five specific works and then slip out unnoticed by the guards. That is quite impressive."
The museum, across the River Seine from the Eiffel Tower, has been cordoned off by investigators.