President François Hollande met Nicolas Sarkozy, his predecessor and opposition leader, on Thursday in the first of several meetings with politicians from across the political spectrum to build unity in response to the attack.
Mr Sarkozy said afterwards: "It was a declaration of war on civilization. Faced with barbarity, civilization must defend itself."
Staff at Charlie Hebdo said the magazine would be published next Wednesday. The newspaper Les Echos reported that €500,000 had been pledged to a fund to keep the publication going with a print run of 1 million — more than 50 times higher than its usual circulation.
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The apparently well-planned attack on the offices of Charlie Hebdo, under police protection for years because of its repeated mocking of Islam and other religions, is one of the deadliest terrorist assaults on European soil in recent years.
It followed months of warnings by the French government of the risk of terrorism from Islamist militants.
European security chiefs are still scrambling to piece together an accurate picture of what exactly occurred on Wednesday and who those responsible are.
In a rare speech due to be given on Thursday evening, Andrew Parker, the director-general of Britain's domestic security service, MI5, will say that it is still "too early for us to come to judgment about the precise details or origin of the attack". He will add that the attack "is a terrible reminder of the intentions of those who wish to harm us".
If confirmed as the work of Islamist militants, it would rank as the worst such terrorist attack in Europe since suicide bombers struck in London in July 2005, killing 52 people.
British government officials said on Thursday that London had tightened border security, including at Calais and the Gare du Nord train station in Paris, where the UK Border Agency has controls. However the government's Cobra emergency committee decided not to raise the terror threat level from "severe", which indicates an attack is highly likely.
According to French media, this is at least the third time Cherif Kouachi has been wanted by the police. In 2005, he was arrested as part of the Filiere des Buttes Chaumont group, named after the leafy park in Paris's 19th arrondissement near where they met to recruit jihadis to fight in Iraq.
A former pizza delivery man, Cherif was detained as he and others in the group prepared to fly to Damascus, Syria. In 2008 he was sentenced to three years in prison, including 18 months he had already served. Cherif said during the trial that he was inspired by the abuse of inmates by US troops at Baghdad's Abu Ghraib prison.
The Paris attack comes against the backdrop of rising political tensions in France and elsewhere in Europe over immigration and the perceived growing influence of radical Islam. Support is growing for the far-right National Front in France, led by Marine Le Pen, which campaigns on these issues. The attack also followed big demonstrations in Germany by Pegida, a new rightwing "anti-Islamization" movement.