At least five drones have been spotted flying over central Paris landmarks during the night and police were unable to catch the operators, a security source says.
The first drone sighting was near the US embassy in the French capital.
The Eiffel Tower, the Place de la Concorde and the Invalides military museum "were also flown over" in the early hours of Tuesday, the source said.
"It could be a co-ordinated action but we don't know for now," the source, who asked not to be identified, said.
"We did everything to try and catch the operators but they were not found," another source close to the investigation said.
A series of mysterious drones have appeared in France during the last few months.
On January 20, an unmanned aircraft briefly went over the presidential palace in Paris, while around 20 drones were earlier seen flying above nuclear power plants.
However until Tuesday "there have never been so many drones appearing in one night," the security source said.
French law bans small, civilian drones from areas such as nuclear facilities, which are protected by a no-fly zone that spans a 2.5-kilometre radius and a height of 1000 metres.