French President Francois Hollande addressed the nation here Wednesday evening, promising citizens security and vowing to pursue and bring to justice the attackers who shot dead 12 people and wounded eight others earlier in the day at the "Charlie Hebdo" satirical magazine.
"France has been attacked in its heart," Hollande said in a brief address.
He declared Thursday a national day of mourning when most civil service offices will be closed and flags will fly at half mast.
Investigators are currently gathering evidence at the headquarters of the weekly magazine and at a second site in the capital, where the perpetrators abandoned a get-away car after a crash with another vehicle.
No claim of responsibility has been made for the attack, but the killers, who used automatic weapons, were recorded on video saying they were linked with Al-Qaeda.
Eight journalists were among the victims of what is being termed a massacre here, and two policemen were also killed, one in cold blood after he was initially wounded.