Thieves have made off with works by Cézanne, Degas, van Gogh and Monet in the biggest art robbery of its kind in Switzerland.
The theft happened on Sunday at the Bührle Collection - a private museum for Impressionist and post-Impressionist art in Zurich.
Three masked men who entered the building with pistols are still at large, police said on Monday, describing the heist as a "spectacular art robbery".
"It's the biggest ever robbery committed in Switzerland and certainly even in Europe," Zurich police spokesman Marco Cortesi told a media conference.
The four works are Cézanne's The Boy in the Red Vest - worth SFr100m on its own - van Gogh's Blossoming Chestnut Branches, Monet's Poppies near Vétheuil and Degas' Count Lepic and His Daughters.
A SFr100,000 reward has been offered for any information leading to the recovery of the paintings.
While one of the men used a pistol to force museum personnel to the floor, the two others went into the exhibition hall and collected the four paintings. The museum was open at the time with around 15 visitors inside the building during the robbery.
The men were about 175 cm tall and one of them spoke German with a Slavic accent, the police said. They loaded the paintings into a white vehicle parked in front of the museum.
"I think they knew exactly what they wanted to steal because it was over in three minutes. They came in and went directly to the right room and took the four most highly valued pictures," Cortesi told swissinfo.
"It is one possibility that they were stolen to order, but what do you want to do with these pictures at home? Everybody now knows these pictures have been stolen."
Museum director Lukas Gloor told journalists that he had not ruled out that a ransom demand would be made, but until now no such communication has been received.
"There is the financial value, but there is also the spiritual value, and we are facing the fact that these paintings are some of the most important in our collection," Gloor told swissinfo.
"We are devastated. I feel like the father of a family who has lost four of his children," he added.
"Regretfully, it has in the past repeatedly been the fact that collectors' museums of this type have been the victims of robberies."
The art collection of Emil Georg Bührle (1890-1956), a Zurich industrialist, is among the most important private collections amassed in the 20th century of European art. In 1960 his family placed 200 works in a foundation and opened it to the public.
The theft comes three days after two paintings by Spanish artist Pablo Picasso were stolen from an exhibition of the artist's works near Zurich.
The oil paintings, believed to be worth several million Swiss francs, vanished on Wednesday evening after closing time at the Seedamm culture centre in Pfäffikon, canton Schwyz. Police are still not sure how the thieves got into the building, but they set off an alarm as they left.
Cortesi told swissinfo that it is too early to say if there is a connection between the two thefts. He added that police are investigating the possibility of inside help, but that there is no evidence at this stage.
The FBI estimates the market for stolen art at $6 billion annually, and Interpol has about 30,000 pieces of stolen art in its database.
While only a fraction of pieces is ever found, the theft of iconic objects, especially by force, is rarer because of the intense police work that follows and because the works are so difficult to sell.
Interior Minister Pascal Couchepin said he regretted the theft and hoped the police would quickly find out who committed the crime and how. Asked whether the museum ought to be better guarded, he said: "Probably, if something like this can happen."
swissinfo, Matthew Allen in Zurich, with agencies
Swiss art heists
At the end of the 1980s three armed robbers made off with 21 Renaissance paintings from a Zurich art gallery. The case was made public in 1989 when FBI agents arrested two Belgians and recovered SFr6.75 million worth of stolen works.
In 1994 seven Picasso paintings worth an estimated $44 million were stolen from the Bollag gallery in Zurich. They were recovered in 2000. A Swiss and two Italians were jailed for the theft.
In 2003 a Swiss court sentenced a French man to four years in prison for the theft of art works in a six-year European crime spree. Stéphane Breitwieser was found guilty of stealing 69 works in Switzerland worth more than SFr1 million. Part of his haul was destroyed by his mother.
On February 6, 2008 two paintings by Spanish artist Pablo Picasso were stolen from an exhibition of the artist's works at the Seedamm culture centre in Pfäffikon, canton Schwyz. The 1962 "Tête de cheval" (Horse's head) and the 1944 "Verre et pichet" (Glass and jug), both on loan from the Sprengel Museum in Hannover, Germany, are believed to be worth several million Swiss francs.
French Impressionism and Post-Impressionism constitute the core of the collection, which also contains the Nabis, Fauves, Cubists and other representatives of the French avant-garde after 1900.
The collection is rounded off by sections devoted to earlier periods, in particular Dutch painting of the 17th and Italian painting of the 16th-18th centuries and a group of Gothic wood sculptures.
Bührle acquired most of his pictures and sculptures between 1951 and 1956. In 1960 his family placed a representative selection of about 200 pictures and sculptures in a foundation and opened it to the public.