At least 13 people died as violent rainstorms turned rivers into raging torrents in southwestern France on Monday in the latest episode of wild weather in Europe, officials said.

Flash floods swamped a number of towns and villages around the fortress city of Carcassonne, leaving a trail of overturned cars, damaged roads and collapsed homes.

An elderly nun was swept to her death as rising waters smashed through a nunnery in the village of Villardonnel to the north of Carcassonne. Meanwhile, at least four people died overnight in the hard-hit village of Villegailhenc, local authorities said Monday.

As Prime Minister Edouard Philippe headed to the scene, the French interior ministry said a total of 13 people had died after the equivalent of three months' of rainfall in just a few hours.

Authorities rushed in helicopters and 600 firemen to help with rescue operations, particularly those in the floodplain of the Aude river which hit its highest level in 100 years, according to the Vigicrues flood agency.

The storms were triggered when a front of warm and humid air from the Mediterranean Sea slammed into colder air around the Massif central mountain range in central France, inundating an area from the eastern Pyrenees to Aveyron further north.

Violent storms on Sunday also hit Portugal, leaving 28 with minor injuries and hundreds of thousands without power amid flooding in the region around the capital Lisbon. (AFP)