A female farmer in her thirties was killed and her husband and teenage daughter were badly hurt when a car crashed into a roadblock during a nationwide protest by French farmers, reports BBC News.
Farmers have cut off key roads for days in the south-west, one of the France's most important agricultural regions.
Early indications suggested the accident south of Toulouse was not deliberate, the local prosecutor said.
Agriculture minister Marc Fesneau said it was a "tragedy for all of us".
Tuesday's accident came as farmers across France expanded their roadblocks after a meeting with Prime Minister Gabriel Attal, to address complaints about new environmental regulations and rising energy costs.
Public prosecutors said the car "struck bales of straw and mowed down three people" while driving on a stretch of dual carriageway on the main N20 national route at Pamiers 70km (43 miles) south of Toulouse, while it was still dark at around 05:45 (04:45GMT) on Tuesday morning.
The farmer who died, named locally as Alexandra Sonac, was described as a cattle breeder from the nearby village of Saint-Felix-de-Tournegat.
The local prosecutor said at one point that the farmer's 14-year-old daughter had died in hospital of her injuries, but later made clear that she remained in a critical condition. The farmer's husband is being treated in intensive care.
The three people in the car were all detained. They had reportedly been driving from Toulouse to Andorra, another two hours' drive to the south.