With France still on high alert after January's jihadist attacks in Paris and yet another audacious jewellery heist in recent days, a huge but discreet security detail is planned for next week's Cannes film festival.

The festival on France's south coast has often been the target of major thefts and this year has proved no exception.

On Tuesday, four men -- one wearing an old-man mask and armed with a pistol -- burst into the Cartier store on the famed Croisette strip and made off with over 17 million euros' ($19 million) worth of jewellery and watches.

The 70-odd luxury stores that line the strip load up on high-value items that they hope will adorn the festival glitterati, making them prime targets for criminals.

In 2013, a man stole diamond-encrusted necklaces and other jewellery worth over 140 million euros from an exhibition at the illustrious Carlton hotel, once the setting of Hitchcock classic "To Catch a Thief". He has never been caught.

For the police, the festival represents 12 long days trying to keep an eye on an enormous crowd, as the population of Cannes triples to some 210,000.

"There are more people and more riches. Thieves come especially for the event," said Cannes police commissioner Philippe Jos.

Beefed-up patrols monitor the surroundings of luxury villas, hotels and boutiques, looking for everything from pickpockets to illegal taxis to more serious robbers.

They are helped by the fact that Cannes is the most densely monitored town in the country, with 468 CCTV cameras -- one for every 152 residents.

"I have received guarantees from the ministry of interior. There will be a special effort behind the scenes -- on intelligence and surveillance," said mayor David Lisnard.

In the wake of the jihadist attacks in January against Charlie Hebdo magazine and a Jewish supermarket in January that left 17 people dead, there is also a heightened vigilance about terrorism.  

Last month saw a plot to attack churches near Paris that was only disrupted because the suspected attacker accidentally shot himself in the leg.

In Nice, half an hour along the coast from Cannes, an extremist tried to attack three soldiers with a knife in February.

"There isn't a particular terrorist threat," said regional prefect Adolphe Colrat, but recent events mean "security has been moved up a level compared to last year."

There is a particular emphasis on coordination this year, with a central command centre for all security forces for the first time.

Alongside 500 police, there will also be four companies of reserve officers, as well as the huge number of private security guards that accompany the stars.

A tight maritime security net has been put in place, drones have been banned from the skies, and the main roads in and out of town are being closely watched.

"The diversity of risks is considerable," said Philippe Castanet, who is in charge of the security plan for the festival.

Organisers have even been monitoring social networks for signs that any kind of demonstrations might be held in Cannes, apparently worried about stars coming across angry protesters.