Bells rang out, Paris public transport stood still and children sat silent as France mourned 12 people killed in an attack on a newspaper office that has rattled the nation.

French President Francois Hollande ordered flags at half-staff and a moment of silence Thursday to honor the victims of the attack on Charlie Hebdo, which had caricatured the Prophet Muhammad. Witnesses said the attackers claimed allegiance to al-Qaida.

Onlookers wept while listening to bells peal at Paris' Notre Dame Cathedral. The rector of the Paris Mosque called on Muslims to observe the moment of silence and honor victims of this "exceptional violence."

Hollande called on the French to unite against terrorism and intolerance.