Ten people suspected of using encrypted social networks to prepare a possible attack were arrested on Tuesday in counterterrorism operations in France and Switzerland, according to French officials.
Searches are still underway in the Paris suburbs and in southeastern France.
The operations were part of an investigation opened in July focused on suspicious activity by a person in Switzerland using the Telegram network, according to a French judicial official.
Counterterrorism investigators detained nine people in France and one in Switzerland in operations aimed at clarifying details of the suspected plot, the official said.
The official, who spoke on condition of anonymity to discuss an ongoing investigation, would not provide details about their identities.
The Swiss-based chief suspect had communicated with people in France on social networks about unspecified violent acts, the official said. A French security official said the suspected plot did not appear to be fully developed but authorities acted Tuesday out of concern that the group was moving toward action.
Among towns targeted in the operation were Aix-en-Provence in southern France and Menton on the Mediterranean coast as well as Paris suburbs, according to the security official.
The operation comes days after a new counterterrorism law came into effect to replace a state of emergency that had been in place since deadly attacks in Paris two years ago.