The robbers disappeared among Carnival revellers

The thieves reportedly brandished a hand grenade to threaten security guards at the Chacara do Ceu museum.

They shut down the security cameras and then slipped away in a crowd of Samba revellers, said museum officials.

Police numbers have been increased in Rio as the five-day celebration officially got under way on Friday.

Disappeared

Pablo Picasso's The Dance, Salvador Dali's The Two Balconies, Henri Matisse's Luxembourg Gardens and Claude Monet's Marine were stolen.

The paintings were considered the museum's most valuable pieces but their exact value was not immediately available.

"They took advantage of a carnival parade passing by the museum and disappeared into the crowd," said museum director Vera de Alencar.

She said the robbery appeared to have been masterminded by specialists, probably from international gangs.

A number of tourists who were visiting the museum at the time were also mugged by the gang.

A nationwide alert has been issued by the federal police to try to prevent the artworks from leaving the country.