Tourists driving at night in the French Rivera have been targeted by robbers posing as policemen, French authorities said, warning motorists to be cautious.

Using an unmarked car with flashlights, the criminals have been pulling over vehicles travelling on highways in France's southern Mediterranean regions in recent weeks.

Following a well-oiled scheme, the disguised robbers stop their victims on the emergency lane to then quickly surround the vehicle, pointing flashlights at the passengers to disorient and intimidate them.

With the excuse of searching for the car for drugs, in a couple of minutes they seize all valuables and also demand to be handed bags, wallets and other belongings, get back to their vehicle and flee.

"They also steal mobile phones and car keys to delay the raising of the alarm," Police captain Alain Archaimbault told France Info.

The robberies have been reported in the coastal regions of Provence-Alpes-Côte d'Azur, home to famed seaside resorts such as Saint Tropez and Cannes, and Languedoc-Roussillon, mostly between two and six in the morning.

A police statement said the gang preferably targeted foreign plated vehicles, exploiting tourists with lesser knowledge of French authorities' uniforms and modus operandi.

Urging motorists to be alert, Archaimbault explained that police do not stop vehicles on highway's emergency lanes for routine controls at night, preferring safer spots as lay-by areas, and do not demand to be given bags and wallets.

In case of emergency or doubt, motorists should dial 17 for police or alternatively 112, the European emergency number.