Rio De Janeiro, Brazil ? For Carnival this year, six young men decided to go as art thieves.

As thousands of Brazilians and foreign tourists donned wigs and masks and costumes to party on the streets outside, a band of robbers executed a daring and violent heist of a Rio de Janeiro museum. They stole four paintings worth up to $50 million: a Monet, a Matisse, a Picasso and a particularly prized work by Salvador Dali.

The theft, which occurred at the Chacara do Ceu Museum just before closing time Friday, is a blow to Brazil’s cultural collection.

“This is very serious, for such an important Brazilian museum to have this loss,” said Christina Penna, a private art curator in Rio.

“I think they knew what they wanted,” said Penna, who worked with the museum when she was curator of a national project to catalog the works of Brazilian artist Candido Portinari. “This is a museum with plenty of things in it, and they went past everything else and went straight for what they wanted.”

The robbers approached the museum as visitors. While two of them waited in a van by the front gate, the other four paid their entrance fee and started toward the galleries.

Then upon announcing the robbery, they brandished guns and at least one grenade. They overpowered the security guards, who worked for a private firm and were unarmed. They herded visitors and museum staff into the security office, where they disconnected the closed-circuit security cameras and ripped out any videotapes that might have captured them on film.

And they collected their loot: Pablo Picasso’s “The Dance”; Dali’s “The Two Balconies”; Henri Matisse’s “Luxembourg Garden”; Claude Monet’s “Marine”; and a book of Picasso engravings illustrating poems by Pablo Neruda.

The works are among the most precious examples of modernist European art that Rio has to offer. The Dali, for example, is the only painting by the Spanish surrealist on public exhibition in Latin America, said museum director Vera de Alencar.

Dancers perform during Vila Isabel samba school parade at the sambodrome in Rio de Janeiro, Brazil, Monday, Feb. 27, 2006.

The robbers ignored the museum’s other works. But they took the time to rob museum visitors, including tourists from New Zealand and Australia. And on the way out, they assaulted a security guard who showed up for a shift change and tried to wrestle the Picasso from one of the robbers.

The robbers then fled through a path in the woods, police and witnesses told the Rio de Janeiro media. Then, taking advantage of the Carnival crowds, they disappeared and, police believe, they escaped in the van.

Jean Boghici, who was a friend of Dali’s and runs a gallery in the Rio de Janeiro neighborhood of Ipanema, said Monday that an unscrupulous collector probably commissioned the robbery. Or perhaps the thieves might try to extort the museum before returning the works.

Selling the pieces on the black market would be difficult, Boghici said.

“We can only hope the bandits turn the paintings over to someone who will take care of them,” Boghici said. “These things are often recovered.”

Indeed, the Dali and the Matisse were stolen once before from the same museum. That happened in 1989, and the works were quickly found.