At least 10,000 soldiers and 8,000 police officers will be deployed across the country, French media reported.
“We must remain vigilant because the threat is still very much present,” Prime Minister Manuel Valls told CNN affiliate BFMTV.
In what one French official said was the largest gathering in French history, 3.7 million people marched at an anti-terrorism rally Sunday.
3,7 million people marched throughout France today. Largest mobilization in history of France. #CharlieHebdo
About 40 world leaders joined in the march.
In Paris, police said no incidents had been reported despite the record number of people involved in the rally there.
‘France without Jews of France is not France,’ French PM says
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The device was thrown into the archive section of the building, setting it on fire. It’s unclear whether the arson attack was connected to the Charlie Hebdo massacre.
The German paper reprinted Charlie Hebdo cartoons depicting the Prophet Mohammed after the attack on the satirical magazine’s offices in Paris.
Anonymous claims it blocked jihadist website
The Anonymous hacking collective has vowed to retaliate for the Charlie Hebdo attack.
“We intend to take revenge in their name, we are going to survey your activities on the Net, we are going to shut down your accounts on all social networks,” a video from the group said Friday.
In online forums Sunday, Anonymous claimed to have blocked at least one jihadist website.
CNN’s Sandra Betsis Ljuljanovic, Barbara Starr, Radina Gigova, Hakim Almasmari, Nic Robertson, Al Goodman, Tim Lister, Faith Karimi, Ashley Fantz, Margot Haddad, Lorenzo Ferrigno, Laurie Segall and Evan Perez and translator Eric King contributed to this report.