Six people have been arrested on preliminary terrorism charges, suspected of plotting to attack French President Emmanuel Macron, an official said.
A judiciary official, speaking on condition of anonymity to discuss the allegations, said intelligence agents detained the six in three widely scattered regions, including the Alps, Brittany and near the Belgian border.
He said the plan appeared to be vague and unfinalised but violent.
Mr Macron is in Verdun on Tuesday as part of First World War commemorations and hosts US President Donald Trump this weekend.
It was reported by BFM TV that the suspects, five men and one woman, are aged between 20 and 60.
The French TV channel also claimed they were members of a far-right movement, but that has not been confirmed by police.
It said the man who is "considered to be the main suspect" was found in possession of a 44-calibre gun.
The suspects have been taken into custody in Isere, Moselle and Ille-et-Vilaine.
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