Journalists have led thousands tweeting the hashtag "Je Suis Charlie" in messages of solidarity with victims of the attack at the Paris office of French satirical weekly Charlie Hebdo.
"Je Suis Charlie" translates as "I am Charlie".
Twelve people were killed when gunmen attacked the building before fleeing in a black Citroën hatchback.
Eight journalists, and two police officers were among those killed, according to Paris prosecutor Francois Molin. Eleven others were injured, some seriously.
The magazine’s editor and chief cartoonist, Stéphane Charbonnier, known as "Charb", had previously defended a controversial series of cartoons published in the magazine depicting the Prophet Mohammed in 2012.
Shocking videos taken by witnesses showed a policeman being shot at close range by two gunmen.
In another, filmed by journalists from the roof of a building close by, shots can clearly be heard in the background. The group filming the exchange are hiding behind a low wall.
President Francois Hollande branded the shooting a terrorist attack and his government raised France's security to the highest level.