France’s President Francois Hollande has ordered security stepped up around public buildings and transport because of military operations in Africa.
Francois Hollande 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, Francois Hollande said. The hostage was believed to have died.
France’s anti-terrorism alert system known as “Vigipirate” is being reinforced immediately, with security boosted at public buildings and transport networks, particularly rail and air. Public gatherings will also be affected.
The alert will remain at red, the second-highest level at which emergency counter-attack measures are put in place.
The “struggle against terrorism” required all necessary precautions to be taken in France itself, the president said.
Francois Hollande’s 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.
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.
A battle erupted with al-Shabab militants and, according to President Francois Hollande, the operation failed “despite the sacrifice of two of our soldiers and probably the assassination of our hostage”.
Earlier, French Defence Minister Jean-Yves Le Drian said one of the soldiers had been killed, another was missing, and “all the indications” were that Denis Allex had been killed by his captors.
But al-Shabab insists the hostage was not in the area at the time of the raid and is alive.
Hundreds of French troops have been deployed in Mali, both in the conflict zone near Konna and in the capital, Bamako.
An estimated 6,000 French expatriates are said to live in Bamako and one of the tasks of the French mission is to guarantee their security.
President Francois Hollande said “heavy losses” had been inflicted on France’s adversaries “but our mission is not over yet”. A Malian army officer said that more than 100 rebels had been killed.
The defence minister said earlier that Paris had decided to act urgently to stop the Islamist offensive, which threatened to create “a terrorist state within range of France and Europe”.
He also revealed that a French helicopter pilot, Lieutenant Damien Boiteux, was killed in Friday’s fighting – during an air raid to support Mali’s ground troops in the battle for Konna.
Eleven Malian soldiers have also been killed and 60 injured in fighting around Konna, Malian interim President Dioncounda Traore said in a statement.
There were reports of civilian casualties too, with Human Rights Watch talking of ten deaths in Konna.
On Saturday, Dioncounda Traore called Francois Hollande to thank him for the operation, the AFP news agency reports.
Dioncounda Traore declared a state of emergency on Friday, which he said would remain in place for an initial period of 10 days.
West African bloc Ecowas has authorized the immediate deployment of 2,000 troops to Mali and UK Prime Minister David Cameron said British forces would offer logistical assistance to help transport troops and equipment.
The Ecowas troops – from Niger, Burkina Faso, Nigeria and Togo – are expected to arrive in the next 10 days.
A government official in Niger told Reuters around 500 troops would be sent while AFP reported that Burkina Faso was sending a similar force.
In a statement, Ecowas Commission President Kadre Desire Ouedraogo said the decision was made “in light of the urgency of the situation”.
For some months, Ecowas had been planning to send 3,300 troops to Mali with the aim of helping government forces reclaim the north of the country.
However, even though the mission was authorized by the United Nations, its deployment was not due to take place until later this year.
Although the French operation appears to have halted the rebel offensive, the logistics are complicated and the task of recapturing northern Mali remains a daunting one.