Police have broken up a terror ring plotting an attack in France after arresting seven suspects in Strasbourg and Marseille,
Interior Minister Bernard Cazeneuve said on Monday.
The arrests of the alleged plotters from France, Morocco and Afghanistan "enabled us to prevent a long-planned terror attack on our soil," Cazeneuve told a televised news conference.
He said the investigation would show whether "the foiled attack was a coordinated attack aiming to target several sites simultaneously on our soil."
Police raids were carried out overnight Saturday to Sunday in the eastern city of Strasbourg and Marseille in the south following an eight-month investigation by security services.
"Credible information made these arrests necessary," one security source told AFP earlier, asking not to be named because they were not authorised to speak on the case.
France remains in a state of emergency that gives security forces enhanced powers to mount surveillance and launch raids, a year after attacks by jihadists on Paris that left 130 people dead.
Security and fears about Islamic extremism are playing a major role in campaigning ahead of France presidential elections due in April and May next year.
"The threat of terrorism has never been so high on our soil," Cazeneuve said Monday. "Zero risk cannot be guaranteed and anyone promising this is lying to the French people."
He linked the plot by the latest suspects, aged from 29 to 37, to raids carried out shortly before the Euro 2016 football championship hosted by France in June and July.
Only one of the detainees was known to security forces, a Moroccan who had been flagged by a "partner country," Cazeneuve said.
The interior minister said 418 people had been arrested for suspected links to terror networks since the start of the year.
Strasbourg has been on high alert ahead of the opening of its Christmas market, with local officials warning it could be suspended or cancelled if there were serious threats.
Election next year
The centre-right Republicans party held the first round of a primary election to select its presidential candidate on Sunday which was won by the conservative Francois Fillon.
As well as promising more police and prison places, 62-year-old Fillon published a book recently called "Beating Islamic totalitarianism."
He will face off against centrist Alain Juppe to clinch the nomination next Sunday, with the winner tipped to meet the far-right candidate Marine Le Pen in the final round of next year's election.
France has suffered three major attacks since January 2015 when gunmen targeted the Charlie Hebdo satirical magazine, followed by last November's atrocities in Paris.
In July, an extremist claiming to be a member of the Islamic State (IS) group drove a truck through crowds watching fireworks in the southern city of Nice, killing 86.
In between, a host of IS-inspired killings, from the murder of a priest in his church to the fatal knifing of a police officer and his partner at home, have deepened fears among the French public.