French anti-terrorism police have arrested seven people in Strasbourg and Marseille and thwarted what the interior minister called a new potential attack.
Bernard Cazeneuve said six of the suspects arrested had not been known to intelligence services.
Mr Cazeneuve said that Sunday's arrests prevented "a terrorist act that had been envisaged for a long time".
The interior minister did not identify the target of the planned attack.
The arrests came five days before the opening of the famed Christmas market in Strasbourg, which attracts tourists from across Europe and was the target of a failed extremist plot in 2000.
Mr Cazeneuve said 43 people have been arrested in November alone as part of anti-terror operations following deadly Islamic State attacks on France over the past two years.
The raids were linked to a series of arrests in June during the European Championship football tournament that was held in cities across France, Mr Cazeneuve said.
Investigators are studying whether it was part of a larger plot to attack multiple sites simultaneously.
The suspects are French, Moroccan and Afghan, and are aged between 29 and 37.
Mr Cazeneuve said six of them had not been known to intelligence services and one was a Moroccan who had been flagged to France by a foreign government.
The raids in Strasbourg reportedly took place in the Neuhof and Meinau neighbourhoods, where authorities dismantled a jihadi network in 2014 that included the brother of an IS bomber who attacked the Bataclan concert hall in Paris last year.
French police have detained 418 people this year in terrorism investigations following deadly IS attacks, Mr Cazeneuve said.