The gems, one a white 47 carat and the other a 15.74 carat blue diamond, were on display at a stall put up by Swiss jewelry label Chopard in the underground shopping arcade at the Louvre, outside the museum's art galleries.
A police spokesman said the employee manning the stand was apparently absent for a few minutes, and that "there was neither any alarm system nor any surveillance camera," at the site.
Denying this, the manager of the antiques show, Christian Deydie, said "there was a little incident and a window was forced open at one of the jewellery displays," Christian Deydie told journalists. "There were six people present and all the security was there, it surely happened in a few seconds."
"If anything there were more staff present than usual as they were expecting a visit from the wife of the Prime Minister, Jean-Pierre Raffarin,' said Chopard co-chairwoman Caroline Gruosi.
"There were a great many people around. It happened in a matter of seconds. These were obviously professionals who broke into the display case. We immediately closed the stand, alerted the police and took fingerprints," she said.
The value and weight of the stolen diamonds given by police were incorrect, she said, but declined to give the correct values.
The Antique Dealers Biennale brings together more than 100 antiques and jewelry dealers who display their rarest and most expensive pieces. Among the items on display was the 128.48-carat "Star of the South", one of the world's most famous diamonds known for its pinkish-brown color, put up for sale by Cartier.
