Paintings by artists including Picasso, Matisse, Monet, Gauguin and Freud have been stolen from a museum in Rotterdam.
Police in the Netherlands said the works were taken from the Kunsthal Museum early on Tuesday morning.
The museum is showing works from the Triton Foundation as part of its 20th anniversary celebrations.
The paintings include Monet's Waterloo Bridge, Picasso's Tete d'Arlequin, Matisse's La Liseuse en Blanc et Jaune and Freud's Woman with Eyes Closed.
Museum spokeswoman Patricia Wessels told the AFP news agency that police were alerted during the night when the alarm went off but that the thief, or thieves, had left the premises by the time police arrived.
Dutch police said the robbery took place at around 03:00 AM (02:00 BST).
Roland Ekkers from Rotterdam Police said: "The alarm system in the Kunsthal is supposed to be state of the art. We've got no reason to believe that it's not but somehow the people responsible for this found a way in and a way out."
It is the biggest art theft in the Netherlands since 20 paintings were stolen from Amsterdam's Van Gogh museum in 1991.
Chris Marinello, director of The Art Loss Register, which tracks stolen artworks, said it was clear some of the most valuable pieces in the collection were targeted.
Marinello said the items taken could be worth "hundreds of millions of euros" - if sold legally at auction. However, he said that was now impossible, as the paintings have already been registered internationally as stolen.
The other stolen paintings are Monet's Charing Cross Bridge, Gaugin's Femme Devant une Fenetre Ouverte, dite La Fiancee and Meyer de Hann's Autoportrait.
"An initial investigation suggests that the robbery was well prepared," a police statement read.
Police are now reviewing videotape footage and calling on any witnesses to come forward.
In a statement, the museum's chairman Willem van Hassel said the museum would be closed on Tuesday.
The Triton Foundation is a collection of avant-garde art and the Kunsthal exhibition was showing its works by more than 150 famed artists, including Paul Cezanne, Salvador Dali, Roy Lichtenstein and Andy Warhol.
Although some of the foundation's works have been on public display in the past, the current exhibition at the Kunsthal was the first time the entire collection had been shown together.
The Kunsthal museum, which means "art gallery" in Dutch, is a display space that has no permanent collection of its own.