At least 17 people have been killed in violent storms and flooding that struck the French Riviera overnight, with three people still missing, President Francois Hollande says.

The president visited the site of the disaster, which occurred when the Cote d’Azur was lashed by up to 180mm of rain in just three hours.

“The toll is not yet final,” Mr Hollande said. “In these moments, we must be fast, efficient and co-ordinated.”

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Three people died when water engulfed a retirement home at Biot near Antibes, and three drowned when their car was trapped by rising waters in a small tunnel at Vallauris-Golfe-Juan.

Other fatalities were reported in Antibes and Cannes.

Rescue teams at Mandelieu-la-Napoule said the water was so murky that they could not see the bodies trapped in underground car parks, where at least seven people died.

“It’s apocalyptic,” mayor Henri Leroy said. “The parking was half-emptied but there are thousands of vehicles. There could be more bodies.”

Fire engines were busy sucking water out of underground parking areas and basements.

Water coursed through Cannes, Nice and Antibes, transforming the streets of three of France’s most glamorous cities into debris-strewn rivers.

“Some cars were carried off into the sea,” Cannes mayor Davis Lisnard said.

Cannes provided emergency shelter for 120 people, Mr Lisnard said.

“We have rescued a lot of people, and we must now be vigilant against looting,” he added.

Damage in the aftermath of violent storms and floods in Biot, southeastern France. Photo: Getty

Mr Hollande issued a message thanking rescuers and local officials and expressed the “solidarity of the nation” to those who had been affected.

About 27,000 homes remained without power early on Sunday, 14,000 of them in Cannes alone.

Communications to the region – one of the wealthiest in France, and a magnet for visitors from around the world – were badly hit.

About 500 people, many of them British and Danish tourists, were stranded at Nice airport.

About a dozen trains were halted at local stations.

The A8 motorway near Antibes was flooded when a small river, the Brague, burst its banks.

Nice’s mayor’s office estimated the city had received 10 per cent of its average annual rainfall in the past two days.

By dawn, the worst storms had passed over the French mainland and were headed for the Italian coast, Meteo-France said.

The region’s worst flood in the past 25 years was in June 2010, when 25 people were killed.

-AAP