Several months’ worth of rain fell in only a few hours over the weekend in Aude, a region in the southwest of France.
One river rose by more than eight metres (26ft) in five hours.
Nine of the 11 deaths occurred in one town, Trebes, in the Aude region, according to Interior Ministry spokesperson Frederic de Lanouvelle.
He added the number of dead could keep increasing, as three people are missing and five others are seriously hurt.
Alain Thirion, the prefect of Aude, told BFM TV: “We have people stranded on rooftops.
“We’re going to have to use aircraft to evacuate them because we cannot reach them by boat given the force of the water. It’s too dangerous.”The flash floods have killed at least four people in the village of Villegailhenc alone, according to local authorities.
“It’s like a war zone. The village is more or less cut off from the outside world,” said Jean-Jacques Garros, a resident of Villegailhenc.
Helene Segura, a 70-year-old who lives in Villegailhenc, added at least one small bridge also collapsed due to the flooding.
She said: “There’s water everywhere in the house. Everything is flooded.
“When I look out the window, I can only see water and mud everywhere.
“It’s sad when you’re 70 years old like me and you need to redo your house, change the furniture and all the upholstery.”
The downpours and floods also hit a region near the medieval hilltop city of Carcassonne and dotted with stone villages on a sweep of land wedged between two mountain ranges andthe Mediterranean.
Mud-swamped village streets were strewn with upended cars and debris after raging rivers broke their banks.
French Prime Minister Édouard Philippe visited the scene on Monday.
The government said it was spending millions of euros to revamp technology at the state weather forecasting agency, after some residents were quoted in the media raising
questions about the speed of its alerts.
Justice Minister Nicole Belloubet said the state would fund a new supercalculator that would help to “fine-tune” weather predictions and would be on line in a year and a half.
Around 600 firefighters were also rushed in to help with rescue operations.
They were needed with lowering the floodplain of the Aude River, as it had reached its highest level in 100 years, according to the Vigicrues flood agency.
Torrential rain often hits France during this time of the year.
However, meteorologists have said warm sea water in the Mediterranean could have intensified following the wake of Tropical Storm Leslie, which formed on September 23.
The remnants of the storm passed over Portugal and Spain this week, but have also wreaked havoc across France.