At around 11.30am, Renault shares were down over 20 percent at €69.02, while rival carmaker Peugeot's stock also fell sharply, by seven percent to €13.70.
Earlier, unions reported that French anti-fraud detectives have raided several production sites of Renault, possibly to investigate emissions foul play.
"Agents from the (anti-fraud unit) DGCCRF intervened in various Renault sites last Thursday," the CGT Renault union said in a tract.
Renault confirmed the raids later on Thursday.
The probe targeted the sites' engine control units which suggests, the union said, that the raids "are linked to the consequences of the Volkswagen rigged-engines affair".
Detectives took several personal computers belonging to Renault managers, the unions said.