A gang of art thieves carried out a well-organized, pre-dawn heist on Monday at Greece's National Art Gallery, the country's largest art museum. They got away with two oil paintings by 20th century masters Pablo Picasso and Piet Mondrian.

A 1939 female bust by Spanish painter Pablo Picasso belonging to the National Art Gallery in Athens, Greece, is seen in this June 30, 2011, handout photo provided by the Greek police.

A police statement said the burglars, who entered through a balcony door, stole two oil paintings and one pen and ink drawing in seven minutes. A fourth works by Piet Mondrian was also removed from the National Art Gallery in one of the best-guarded areas of central Athens, but it was abandoned as the burglars fled. They triggered a sensor in the exhibition area, and a guard only got there in time to see a man running off.

Among the stolen works was a cubist female bust by Pablo Picasso, which the Spanish painter had donated to Greece in 1949 with a dedication "in homage to the Greek people" for their resistance to Nazi occupiers during World War II.

The thieves also took a 1905 representational oil painting of a riverside windmill by Mondrian, famous for his later abstract linear works, and a drawing of St. Diego de Alcala by Caccia.

The art gallery contains mostly 19th and 20th century Greek paintings. Museum officials were unable to immediately estimate how much the stolen works were worth. The Gallery had been due to close on Monday for a long period of extension and refurbishment.