"There are 3,000 Europeans in Iraq and Syria today. When you do a projection for the months to come, there could be 5,000 before summer and 10,000 before the end of the year," Valls told French television channel iTele. "Do you realise the threat that this represents?"
About 1,400 people were in conflict zones with organisations such as the Islamic State armed group as he spoke, Valls said.
"There have already been nearly 90 French people who have died out there with a weapon in their hand, fighting against our own values."
Last month France seized passports from six of its citizens and banned 40 more from travelling abroad after they were allegedly planning to travel to Syria and Iraq.
With regional elections later this month in which his Socialist Party is expected to do poorly, Valls sounded the alarm over the FN’s good showing in opinion polls.
"Do you not think that an FN which wins 25 percent in European elections and maybe 30 per cent in local elections ... cannot win the presidential election?" he asked.
FN leader Marine Le Pen should be “stigmatised”, he said.
"I am scared for my country, I'm scared that it will crack in the face of the Front National," he declared, calling for voters to turn out to reduce the FN’s share of the vote.
The FN warned that Valls's statement "cannot help but have consequences on his political future" in a statement on Sunday afternoon.