Local television showed black-clad sharpshooters from the elite police unit in the streets of Reims, in France's Champagne region, as unconfirmed media reports named three suspects in the attack, including two brothers.
Several thousand police were deployed to find the gunmen and parts of the French capital were in lockdown as the killers remained on the loose.
In a sombre televised address, French President Francois Hollande declared a day of national mourning on Thursday- only the fifth in the past 50 years- after the worst attack on French soil in decades.
The Paris attack on Charlie Hebdo, a magazine that has long been in confrontation with Islamists, triggered impromptu demonstrations of solidarity in cities across the world, including Moscow, Washington, London and Tokyo.
More than 100,000 gathered across France, many protesters carrying banners reading: "I am Charlie" while the hashtag #JeSuisCharlie was trending worldwide including in Arabic.
"Nothing can divide us, nothing should separate us. Freedom will always be stronger than barbarity," said the President, who ordered flags flown at half-mast for the next three days.
Charlie Hebdo gained notoriety in February 2006 when it reprinted cartoons of the Prophet Mohammed that had originally appeared in Danish daily Jyllands-Posten, causing fury across the Muslim world.
The killers on Wednesday screamed "we have avenged the prophet, we have killed Charlie Hebdo", according to prosecutors.
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