France's president has urged the nation to stay united as it hunts gunmen who killed 12 people at the office of a satirical newspaper.

Speaking on national television Wednesday night about the slain journalists, Francois Hollande said, "They are today our heroes."

The attack at the offices of Charlie Hebdo weekly newspaper was France's deadliest attack in decades. The newspaper has caricatured Islamic extremists and the Prophet Muhammad.

The attackers are at large, and Hollande called for them to be chased down, judged and punished.

Hollande said France would hold a national day of mourning and fly flags at half-staff for three days.

France has seen a rise in religion-related tensions. Hollande told the French "Let us unite ... and we will win."