Agen in the Lot-et-Garonne département recorded 129.6mm of rain in less than three hours, the highest ever recorded in the town, according to French weather forecaster Météo France.
The rainfall led to flooding 2m deep in streets, homes and cellars.
Local fire and rescue chiefs said that 300 firefighters had been called to the town and 20 people rescued, adding that there were no reported injuries.
“I lost everything in my garage, my car was covered right up to the engine, I’ve never seen flooding like this. I’ve already had 10cm of water, but this is the first time I’ve had a metre,” local resident Yves, whose garage was completely flooded, told French TV channel BFM.
“It happened very quickly, in ten minutes, everything was flooded,” added Mathieu, who saw almost a metre of water in his house.
A total of 32 départements had been placed on alert for storms on Wednesday as the late summer heatwave broke in dramatic fashion, with a large band of storms and heavy rain passing over the centre of the country.
#vigilanceOrange 🟠 ⛈ Des orages, souvent très forts, se déclenchent sur une ligne allant du Gers jusqu'au Centre. De 30 à 60 mm en 1h avec de la grêle localement.
⚠️ Soyez prudents et restez informés ➡️https://t.co/w5OGXbEEhP pic.twitter.com/Z3rgWOiDJk
— Météo-France (@meteofrance) September 8, 2021
More than 3,000 lightening strikes were detected in Lot-et-Garonne on Wednesday evening, according to l’Observatoire français des orages et tornades (French observatory of storms and tornadoes) while in neighbouring Dordogne firefighters also reported flooding and falling trees.
Between 200 and 300 households were also left without electricity.
Météo France said that although the storms had largely passed over, bad weather would persist on Thursday in parts of the south west.