Volunteers and firefighters began the gruelling task Monday of cleaning up French Riviera towns strewn with mud and debris, as the death toll from floods which tore through the region rose to 20.

Citizens of Britain, Italy and Portugal were among those who died when a torrential weekend downpour trapped people in garages and retirement homes in Mediterranean resort towns beloved by jet-setting tourists.

Water gushed through the streets of Cannes, Nice and Antibes, leaving cars including Porsches and Ferraris scattered about, often upside down and hundreds of metres from where they were parked.

Another person was found dead Monday in the worst-hit town of Mandelieu-la-Napoule, with eight now confirmed killed there after being trapped in garages when they tried to remove their cars, a local official said.

And in Cannes a body was found in an underground parking lot. Two people who had been listed as missing were found alive.

Mandelieu-la-Napoule mayor Henri Leroy on Sunday described the situation as "apocalyptic".

In London, the Foreign Office said a British national had died, while an Italian woman and Portuguese man were also among the dead, a local official said.

As night fell Monday, officials said two German citizens were missing in Antibes.

In the town of Biot, about 40 kilometres (25 miles) from Monte Carlo where three people drowned in a retirement home, hundreds of clean-up workers clad in diving suits and carrying chainsaws removed trees and debris strewn around a river.

As the Mediterranean sunshine returned, volunteers grabbed mops to help victims of the damage clean out thick layers of mud from their homes and businesses.

"It is everywhere, it smells bad and it seeps into everything. It is making the furniture swell up," said one local woman, who did not give her name.

Some 9,000 households in the region still had no electricity after record rainfall that saw 180 millimetres (seven inches) of water fall on Cannes alone -- nearly two months' worth in three hours.

- An environmental lesson -

President Francois Hollande toured the region, where he said a state of natural catastrophe would be declared to allow emergency funds to be channelled to the devastated region.

French weather service Meteo France came under fire over the fact the region had been placed only on "orange alert" status ahead of the storm, short of the top level of readiness.

Pascal Brovelli of the Meteo-France weather service told AFP the damage was due to more rain than expected falling in a very short period of time in heavily urbanised areas and in small area.

"Should we have been (on red alert)?

"We will have to see if we could have anticipated what happened," government spokesman Stephane Le Foll said on France Inter radio.

Hollande said Sunday an environmental lesson should be learnt.

"There have always been catastrophes. But their frequency and intensity are on the increase," he said, urging that environmental decisions "be taken" as France prepares to host UN-led climate talks in December on a post-2020 pact to curb greenhouse gases.

- 'Waist-height in 5 min' -

Also hard-hit was Antibes Marineland, Europe's biggest marine park, where penguins and sea lions found themselves in mud-filled pools.

"We have no electricity, no flowing water," said park director Bernard Gianpaolo, as dozens of firemen worked to pump out water blocking filtration systems.

Shocked residents gave graphic accounts of the drama.

"I saw water pour in from the veranda. Within five minutes, it was up to my waist," said France Oberlin, a retired resident of Mandelieu-la-Napoule.

Guy Morales in Cannes described how he was in his car when "a sort of wave carried us away. We couldn't do anything, the car hurtled down the street."

He, his wife and his dog managed to get out of the car and waited on top of a wall for three hours for the water -- which had been at shoulder height -- to recede.

Rail operator SNCF was trying to restore train services after serious damage to tracks, electrical lines and infrastructure in the floods.

Interior Minister Bernard Cazeneuve said nine people had been arrested in Cannes for looting.

The region's worst flood in the past 25 years was in June 2010, when 25 people were killed and one billion euros ($1.12 billion) of damage was caused.

In December 1999, 92 people in France were killed by flooding and fallen trees as hurricane-strength winds battered northwestern Europe.

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