French President Francois Hollande announced Thursday would be a day of mourning in the wake of the attack on satirical weekly Charlie Hebdo that left at least 12 dead.

It marks only the fifth time in the last 50 years that a day of mourning has been declared in France.

"Unity is our best weapon," Hollande said in a televised address, adding that flags would fly at half-mast across the country for three days.

"Nothing can divide us, nothing should separate us. Freedom will always be stronger than barbarity."

The president praised the "courageous chroniclers" of Charlie Hebdo, a magazine which sparked outrage in the Muslim world after publishing controversial cartoons of the prophet Mohammed.

Eight of its journalists, including four of the country's best-known cartoonists, were killed in the attack at its office in central Paris earlier in the day.

"They touched -- by their influence, by their insolence, by their independence -- generation after generation of French," Hollande said.

"This message of freedom, we will continue to defend it in their name."