The French government has placed the country on the highest state of terror alert after an attack on the offices of satirical magazine Charlie Hebdo left 12 dead, including two police.
Extra security forces were drafted in to secure mosques, synagogues, department stores, shopping centres, train stations and airports, Prime Minister Manuel Valls said.
Paris announced that an extra 650 soldiers and 2,000 more police officers will patrol the streets of the capital on Thursday.
The cabinet held an emergency meeting to come up with a response to the deadly attack, while President François Hollande, who has called for "national unity", planned a nationally televised address.
Hollande said that "several terrorist attacks had been foiled in recent weeks".
France last year beefed up its anti-terrorism laws and was already on high alert after repeated calls from Muslim extremists to attack its citizens and interests in reprisal for French military strikes on Islamist strongholds in the Middle East and Africa.
Fears of a wave of attacks were heightened in the run-up to the Christmas holiday period after three separate attacks across France - all involving men shouting Allahu Akbar (God is great) - left one dead and several injured.
Thursday’s gun attack on Charlie Hebdo magazine, which had already been firebombed in 2012 after it published satirical cartoons depicting the Prophet Mohamed, came as politicians in France and other European countries warned that their countries could suffer from fallout from the wars in Iraq and Syria.
Around 1,000 French nationals and hundreds of other European citizens have gone to the Middle East to wage jihad alongside the radical Islamic State group, and many worry some of them will return to carry out attacks in their home countries.
Muslim community fears a backlash
The Charlie Hebdo attack has also raised fears of a backlash against Muslims in France, which has the largest Muslim community in western Europe.
It also comes at a time when there anti-Islam sentiment is growing across Europe and the question of Muslim immigration in France is once again bubbling away.
Wednesday's shooting coincided with the release of a new novel by France's most famous polemicist Michel Houllebecq, who once described Islam as a cretinous religion.
His novel "Submission" projects a future France under Islamic rule and has been blasted by critics as Islamophobic.
Wednesdays shooting will have many in the Muslim community on edge.
"After the victims of this attack, the next victim will certainly be a Muslim," Samy Debah, president of the president of the Collective Against Islamophobia in France (CCIF), told The Local.
"Muslims in France have nothing to do with these attackers. We are not responsible, neither individually or collectively,” he said, adding that he condemned the latest attack, one of the deadliest in France in decades.
Other leaders of the Muslim community firmly condemned the attack and echoed Hollande's calls for unity.
The imam of one of France's major cities, Bordeaux, urged Muslims to take to the streets in protest at Wednesday's deadly attack in Paris by Islamist gunmen, calling it "almost an act of war".
Imam Tareq Oubrou, a supporter of dialogue between Christians and Muslims, said after meeting Pope Francis that the attack on satirical weekly Charlie Hebdo, in which 12 people died, was "tantamount to what September 11 meant to America."
"With this drama we are seeing almost an act of war," Oubrou told reporters.
The current rector of the Grande Mosque in Paris Dalil Boubakeur said: "This attack is against all our values. We are absolutely horrified and stupified by this crime. We are entering an extremely dangerous situation in Paris when violence is becoming a part of daily life," he said.
The Imam of Drancy, Hassan Chalgoumi, added: "I am very angry. We must avoid at all costs equating these barbarians with the majority of Muslims.
"If you don't agree with Charlie Hebdo cartoons then you should respond with your own cartoons, not with blood and hatred.”