Dutch police say thieves have stolen seven paintings by prominent modern artists, including Picasso, Matisse and Monet, from a museum in Rotterdam.

The thieves removed the masterpieces from the Kunsthal museum before dawn Tuesday, triggering an alarm but leaving the scene before police arrived. The art theft is one of the biggest in the Netherlands in recent years.

Authorities said they were reviewing the museum's videotapes and appealing for witnesses as part of an investigation into what they called a well-planned operation.

Art experts said the paintings would be worth tens of millions of dollars if sold at auction. But they said it would be nearly impossible for the works to be sold on the open market because they are so well-known and have been registered as stolen. Some said the thieves may demand a ransom.

The seven paintings include Pablo Picasso's “Harlequin Head,” Henri Matisse's “Reading Girl in White and Yellow,” Claude Monet's “Waterloo Bridge, London” and “Charing Cross Bridge, London,” Paul Gaugin's “Girl in Front of Open Window,” Meyer de Haan's “Self Portrait,” and Lucian Freud's “Woman with Eyes Closed.”

The works were part of an exhibition of 150 paintings from the Triton Foundation collection, privately owned by the Cordia family of the Netherlands. The exhibition marks the 20th anniversary of the Kunsthal museum, a display space that has no permanent collection. The museum was closed to the public following the theft.