Two armed men have stolen eight engravings by French artist Matisse and at least another five by Brazilian painter Cândido Portinari from a library in São Paulo.
Brazilian officials say the thieves held up a security guard and an elderly couple who were visiting the library before making off with the artworks on foot.
They reportedly entered the library by the main entrance at 10:00 (13:00 GMT) on Sunday, and left by the same route, heading towards the nearest metro station.
The heist comes less than two months after the art world was rocked by a brazen break-in at the Louvre museum in Paris, where thieves made off with priceless jewels.
The engravings stolen from Biblioteca Mário de Andrade on Sunday formed part of a joint exhibition with the São Paulo Museum of Modern Art.
The thieves targeted the exhibition, entitled From Book to Museum, on its final day.
Bibilioteca Mário de Andrade is the country’s second largest library and officials say that its building in the centre of São Paulo had cameras with facial recognition technology.
The mayor of São Paulo told local media the thieves had already been identified but so far they remain on the run.
Officials have not yet released a detailed list of the stolen works, but according to Folha de São Paulo newspaper, a collage Matisse made for the limited-edition art book Jazz is among those taken.
Matisse is widely considered to be one of the 20th Century’s most influential artists and art critics say the value of the stolen works is “incalculable”.
The thieves also took at least five engravings by Portinari, created to illustrate a special edition of the novel Menino de engheno (Plantation Boy) by Brazilian writer José Lins do Rego.
Portinari, who often painted rural workers and labourers, is one of the most significant Brazilian Modernist artists.
