A major manhunt has been launched in here for three gunmen who shot dead 12 people at the office of the French satirical magazine Charlie Hebdo, media reported.
Eight journalists, including the magazine's editor, and two policemen were among the dead.
Protests over the killings are being held in cities across France. It was the country's deadliest attack in decades.
President Francois Hollande called it a "cowardly murder" and declared a day of national mourning Thursday.
He said the country's tradition of free speech had been attacked and called on all French people to stand together. "Our best weapon is our unity," Mr Hollande said in a televised address late Wednesday.
Security has been stepped up across France in the wake of the attack, with Paris placed on the highest alert.
The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was fire bombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.
