"Those thieves got one hell of a haul," said Chris Marinello, who directs the Art Loss Register.
The heist at the Kunsthal museum is one of the largest in years in the Netherlands, and is a stunning blow for the private Triton Foundation collection, which was being exhibited publicly as a group for the first time.
"It's every museum director's worst nightmare," said Kunsthal director Emily Ansenk, who had been in Istanbul on business but returned immediately.
News of the theft "struck like a bomb," she said at a press conference in the museum's cafe.
She declined to reveal any details of how the thieves got in and out with the paintings, or how the museum is protected, other than describing its security as "state of the art" and "functional."
Willem van Hassel, the museum's chairman, said its security systems are automated, and do not use guards on site.
Police arrived at the scene five minutes after an alarm was triggered, he said. He described the museum's insurance as adequate for the exhibition.
The collection was on display as part of celebrations surrounding the museum's 20th anniversary.
Police spokeswoman Willemieke Romijn said investigators were reviewing videotapes of the theft, which took place around 3 a.m. local time. She called on any witnesses to come forward with information.
The Art Loss Register's 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.
Interpol sent a bulletin alerting member countries to the theft, along with images of the stolen paintings.
They were: Pablo Picasso's 1971 "Harlequin's Head"; Claude Monet's 1901 "Waterloo Bridge, London" and "Charing Cross Bridge, London"; Henri Matisse's 1919 "Reading Girl in White and Yellow"; Paul Gauguin's 1898 "Girl in Front of Open Window"; Meyer de Haan's "Self-Portrait," around 1890, and Lucian Freud's 2002 work "Woman with Eyes Closed."
Marinello said the thieves have limited options available. They may try to seek a ransom from the owners, the museum or the insurers. They could also conceivably sell the paintings in the criminal market — but only for a fraction of their true worth.
He said the idea that an unscrupulous private investor might have commissioned the works' theft was far-fetched.
"That's something that comes from Hollywood movies," he said.
The Triton Foundation is a collection of avant-garde art put together by multimillionaire Willem Cordia, an investor and businessman, and his wife, Marijke Cordia-Van der Laan. Willem Cordia died last year.
Asenk said she spoke on behalf of the family in saying "we are shocked, but we will go on."
"All involved want the public to still be able to see these kinds of special collections and private collections," she said.
The museum was cordoned off as police carried out their investigation Tuesday, but it will reopen Wednesday, she said.
The Kunsthal museum is a display space that has no permanent collection of its own — the name means "art gallery" in Dutch.
The Cordia family collection includes works by more than 150 famed artists. Others whose work was on show include Paul Cezanne, Marc Chagall, Salvador Dali, Edgar Degas and Andy Warhol.
Curators of the Cordia family collection aim to have the works on display for the public, and the pieces have been shown individually or in small groups in the past.