A robber brandishing a pistol and a grenade stole diamonds worth 15 million euros in the French Riviera town of Cannes and calmly escaped on foot, a regional prosecutor said.

The man in his 30s, his face uncovered except for a pair of sunglasses, entered a luxury jewellery shop on the city's famed Croisette waterfront promenade and initially passed himself off as a customer.

"He asked to see diamond ornaments on behalf of a sponsor. The saleswoman was suspicious and gave him a catalogue," the prosecutor said.

He then pulled out a 7.65-calibre automatic pistol and a grenade and ordered the two saleswomen to open the display cases, the prosecutor said. The investigators did not know whether the grenade was real.

One of the guards, who was nearby, was forced to lie down. Nobody was injured during the hold-up, which lasted about eight minutes, a source close to the investigation said.

It is not the first time that the Croisette has been hit by an audacious robbery. In July 2013, a man stole 103 million euros of jewels from a temporary exhibition at the Hotel Carlton. Neither he nor the jewels have been found.