Water levels rose to 1m20 in the streets of Molières-sur-Cèze, a town of 1,700 inhabitants near Alès in the Gard region.
Muddy water ran through the streets on Monday morning and car parks and basements flooded.
The town's retirement home, home to 70 senior citizens, has also been damaged.
“I don’t know what to do, where to start and how to deal with the situation”, Mayor Georges Adryanczyk-Perrier said.
Cars were washed away with passengers in in the village of Allègre-les-Fumades close to Alès.
Firefighters conducted more than 170 emergency interventions over the weekend but nobody has been injured, according to police sources.
Heavy rain also hit five other regions - Vaucluse, Bouche-du-Rhône, Isère, Drôme, Ardèche - on Sunday, with cumulative rainfall of up to 208mm in some places.
A storm and flooding alert was lifted on Monday morning although strong winds were expected on the Atlantic coast and the Channel coast.
Thunderstorms were expected to move to the eastern part of the country.