(CNN) -- Swiss police were scrambling Monday in search of three masked men who stole four Impressionist paintings worth about $163 million (180 million Swiss francs) Sunday in a heist police characterized as "spectacular."

The Emil-Georg Byhrle private art foundation in Zurich from which works by Cezanne, Degas, Monet and Van Gogh were stolen.

The three men entered the E.G. Buehrle Collection -- among the finest collections of Impressionist and post-Impressionist art in the world -- in Zurich, Switzerland, at around 4:30 p.m. CET (8:30 a.m. ET), police said.

One of the men threatened personnel at the museum's front door with a pistol and forced them to the ground, police said, while the other two men went into an exhibition room and stole four oil paintings by Paul Cezanne, Edgar Degas, Claude Monet and Vincent van Gogh.

Afterward, the three men loaded the paintings -- Monet's "Poppies near Vetheuil," Degas' "Count Lepic and his Daughters," Van Gogh's "Blossoming Chestnut Branches" and Cezanne's "Boy in a Red Vest" -- into a white car parked in front of the museum and then drove off, police said.

Police said the men were wearing dark clothes and hoods, and one of them spoke German with a Slavic accent. They were all of average height, police said.

There is a reward of $91,000 (100,00 Swiss francs) for information leading to the return of the paintings, police said.

The Swiss art heist follows the recent theft in Switzerland of two paintings by Pablo Picasso, Bjoern Quellenberg, a spokesman for the Kunsthaus, a major art museum in Zurich, said.

The director of the Kunsthaus serves on the E.G. Buhrle private art foundation's council, Quellenberg said.

In that theft, thieves stole the paintings, the 1962 "Tete de Cheval" ("Horse's Head") and the 1944 "Verre et Pichet" ("Glass and Pitcher") by Picasso. They were on loan from a German museum and valued at $4.5 million when they were stolen February 6, according to news reports.