French president Francois Hollande has ordered security step up around public buildings and transport areas because of military operations in Africa.
He was responding to the risk of Islamist attack after French forces attacked militants in Mali and Somalia.
A pilot was killed as air strikes were launched on Malian rebels.
In Somalia, two French soldiers were “sacrificed” in a raid to free a French hostage who was eventually killed.
His remarks came within hours of one of the Islamist groups targeted by French military action in Mali threatening reprisals against France.
An Ansar Dine spokesman told Reuters news agency, there would be consequences for French citizens throughout the Muslim world.
The operations in Mali and Somalia were launched within hours of each other but were “totally unconnected”, according to government officials.
French troops were deployed in Mali on Friday after the army lost control of a strategically important town to Islamists who were advancing south. The rebels took control of a huge swathe of northern Mali last April.
