French President Francois Hollande has ordered security stepped up around public buildings and transport because of military operations in Africa, reports the BBC.
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 a column of Malian rebels.
In Somalia, two French soldiers were "sacrificed" in a raid to free a French hostage, Mr Hollande said. The hostage was believed to have died.
France's anti-terrorism alert system known as "Vigipirate" is being raised to boost security at public buildings and transport networks.
The "struggle against terrorism" required all necessary precautions to be taken in France itself, the president said.
Earlier, one of the Islamist groups targeted by French military action in the west African state of Mali threatened 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.
The central town of Konna has since been recaptured, the Malian government says.
Then, French commandos went into action in Somalia, swooping on the town of Bulo Marer in an attempt to free Denis Allex, who was kidnapped in July 2009.
Read more of this report from the BBC.