The head of France's internal intelligence services the DGSI released the figures to highlight that despite terror group Isis facing defeat in the Middle East the danger of homegrown terrorism in France remains high.
Laurent Nunez told RTL radio that the names of 18,000 individuals are on a list of people flagged up for being radicalized towards violence - a steep rise on the number a year ago when it stood at 15,000.
France's various intelligence agencies are keeping tabs on those 18,000 names on the FSTRP list (Fichier des signalements pour la prévention de la radicalisation à caractère terrorist).
Nunez said the DGSI have placed around 4,000 individuals who intelligence services believe are the most dangerous under tighter surveillance. He did not reveal details of how exactly intelligence services were keeping watch on these potentially dangerous people.
"Our chief role is to prevent attacks, detect threats and neutralize them as soon as possible," said Nunez.
While Isis might have lost much of its territory in the Middle East including its de facto "capital" in Raqqa Nunez pointed out there were still individuals in France who could "respond" to its propaganda.
Last week France's specialist terrorist prosecutor François Molins noted that 2017 had seen a rise in "isolated attacks" - 11 in total, including one at Marseille train station that left two young students dead.
Since 2013 some 46 terror plots have been foiled in France while 17 failed.
Nunez said the likelihood of commando teams being sent from the Middle East to attack Paris as was the case for the November attacks two years ago was less likely but could not be ruled out.