At its peak the storm dumped 150mm of rain in the space of just two hours, left packed restaurants and hotels flooded and parts of the Riviera coast without power.

The victims included three people killed at a retirement home that was flooded in the village of Biot, and seven inhabitants of the town of Mandelieu-la-Napoule who had attempted to reach their cars in a garage, local government officials said.

Five people were still reported missing on Sunday afternoon, which could push the death toll higher, officials said.

In the resort town of Cannes, home to the prestigious annual film festival, two people were killed, the train station was flooded, and cars were swept along streets by torrents of rain.

Visiting the affected zone on Sunday, President Francois Hollande said the government would approve natural disaster status for the area at Wednesday's cabinet meeting and pledged relief payments to help cover damage within three months.

Residents were taken by surprise by the exceptionally violent downpour over Alpes-Maritimes, which lies at the eastern end of France's Mediterranean coast and borders Italy.

The Alpes-Maritimes district of France, in which the Riviera is located, said rain of "exceptional scale" had caused widespread damage in the region.

#flood #flooding #Cannes #tonight The streets all surrounding the #Palais were like #rivers 2ft+ deep in some places pic.twitter.com/hNOVcqJwQL— John Henry Canavan (@JHCanavan) October 3, 2015

The severe storms also closed the region's only major airport, Nice-Cote d'Azur, forcing aircraft to be diverted to nearby cities, including Marseilles and Lyon. The airport was re-opened to traffic after midnight Saturday, local time.

According to reports, parts of the railway line which services the Riviera, from St Tropez to Nice, and passing through Cannes and Antibes, have been closed due to flooding.

The flooding had killed least 16 people, officials told Reuters on Sunday. French President Francois Hollande had earlier said there had been at least 12 deaths.

The first victims were three people killed at a retirement home that was flooded in Biot, local officials said.

Authorities said that parts of the coast were inaccessible due to flooding and it was proving difficult to get accurate information.

Cannes city pic.twitter.com/T9EwRe0Rp9— ems (@datsemy) October 3, 2015

The storms hit on the eve of the annual television market Mipcom, which has brought almost 14,000 television executives, producers and personalities from around the world to Cannes.

It is due to begin Monday, local time, but delegates have been arriving by the plane-load since Friday.

Among this year's guests are the newly anointed Top Gear host Chris Evans, The X-Files creator/producer Chris Carter, Heroes Reborn creator Tim Kring and British screenwriter Andrew Davies.

Australian television presenters Kris Smith and Brendan Moar are also attending the market; both are presenting programs for a new European television channel.

And a large contingent of Australian executives is attending, including programmers from Australia's commercial, public and pay TV networks, and key producers.

Seriously though, this is the state of Cannes' main thoroughfare right now pic.twitter.com/xk2V429l9X— Stuart Dredge (@stuartdredge) October 3, 2015

Despite its relatively tiny size, Cannes is a popular location for the broadcasting and media industry's key conventions, including Mipcom, MipTV, the Cannes Lions and the Cannes Film Festival.

Heavy rain had been forecast for the weekend across the French Riviera, including Cannes, as well as the western coast of Italy.

The weather system affecting the region was described by European media as a "medicane" storm.

A "medicane" storm is a tropical-like cyclone which forms in the Mediterranean Sea; it is described as having similar characteristics as an Atlantic Ocean subtropical system.

Though they are not naturally tropical storms, the warm water of the Mediterranean Sea helps the storm take on tropical characteristics.

After the Flood in #cannes pic.twitter.com/pXevkMGkgz— Dr Médard de Chardon (@DocteurVMDC) October 4, 2015

The term "medicane" is a portmanteau word, formed from "Mediterranean" and "hurricane"; hurricane is the term used to describe cyclones which form in the Atlantic ocean.

Local media reported that around 30mm of rain fell in the 24-hour window preceding the worst part of the storm along most of the coastal towns of the French Riviera.

The highest level of rain recorded was at Cannes, which was hit with 37mm of rain in the 24-hour period before the storm hit, the same amount of rain which falls across a week at this time of year.  

More - #Cannes flooding pic.twitter.com/lk1dMmUQiZ (via @janinthesky )— Gregor Peter (@L0gg0l) October 3, 2015