A staggering 40 million euro (£34 million) worth of diamonds and other jewels have been stolen from the Carlton Intercontinental Hotel in Cannes, in one of Europe's biggest jewellery heists in recent years.
One expert noted the crime follows recent jail escapes by members of the notorious "Pink Panther" jewel thief gang.
The hotel in the sweltering French Riviera was hosting a temporary jewellery exhibit over the summer from the prestigious Leviev diamond house, which is owned by Israeli billionaire Lev Leviev.
A police spokesman said the theft took place around noon, but he could not confirm local media reports that the robber was a single gunman who stuffed a suitcase with the gems before making a swift exit.
The luxury Carlton hotel is on the exclusive Promenade de la Croisette that stretches a mile and a half along the French Riviera, and is thronged by the rich and famous throughout the year. The hotel's position provides not only a beautiful view of the sea but also an easy getaway for potential jewel thieves along the long stretch of road.
"It's a huge theft. Anytime you talk about a heist with many millions of dollars it turns heads and feeds the imagination," said Jonathan Sazonoff, US editor for the Museum Security Network website and an authority on high-value crime.
He said the likelihood of recovering the stolen diamonds and jewels is slim, because the thieves can easily sell them on. "The fear is, if you're dealing with high-quality minerals, it's hard to get them back," Mr Sazonoff said. "They can be broken up and so they can be easily smuggled and sold."
The valuable gems were supposed to be on public display until the end of August. It was not immediately clear how many pieces were stolen.
Several police officers were placed in front of the Carlton exhibition room - near a Cartier diamond boutique - to prevent the dozens of journalists and photographers from getting a look at the scene of the crime. Hotel officials would not comment, and attempts to get comments from Leviev or his company were not immediately successful.
Cannes appears to be a favourite target for jewel thieves this year - in May it was struck by other two highly publicised jewellery heists during the Cannes Film Festival. In the first theft, robbers stole about one million dollars (£650,000) worth of jewels after ripping a safe from the wall of a hotel room. In the second, thieves outsmarted 80 security guards in an exclusive hotel and grabbed a De Grisogono necklace that creators said is worth two million euros (£2 million).