An exhibition featuring diamonds from jeweller Leviev was being held in the lobby of the Carlton Hotel in Cannes when thieves struck, making off with an estimated $53 million (£34 million) haul in what is allegedly the second-largest heist in France.
A Cannes police spokesman said, "A full and urgent operation is under way to catch the culprit and recover these jewels. Thieves see Cannes as rich pickings."
The plush hotel provided the setting for Alfred Hitchcock's 1955 film about a cat burglar prowling the French Riviera and is also where the movie's star Kelly met her future husband Prince Rainier of Monaco.
The Cannes Film Festival was hit by a spate of jewellery thefts earlier this year (13).