Six people, linked to the far-right, were detained on Tuesday in connection with a "violent" plot against French President Emmanuel Macron, officials said.
The six, who were not identified, were picked up by anti-terrorism units in the eastern French regions of Moselle and Isere and the northern region of Ile-et-Vilaine, officials added.
The arrests followed reports of a possible plot involving "violent action" aimed at the president, a source close to the investigation said.
Prosecutors have launched an investigation into a "criminal terrorist association", a judicial source said.
"This investigation is looking into a plot, vague and ill-defined at this stage, involving violent action against the president of the republic," another official said.
In July 2017, a 23-year-old far-right extremist was charged with plotting to assassinate Macron at France's Bastille Day military parade which the French leader attended with US President Donald Trump.
The man, arrested at home, told investigators he wanted to kill Macron at the July 14 national day parade in Paris, along with "Muslims, Jews, blacks and homosexuals."
Three kitchen knives were found in his car and analysis of his computer found that he had conducted internet searches as part of his plot.