Hollande said that the multiple attacks across the city late Friday were "an act of war... committed by a terrorist army, the Islamic State, against France, against... what we are, a free country."
Hollande also declared three days of national mourning after describing the terror attacks as acts of "absolute barbarity".
Meanwhile, European capitals have reinforced security checks following attacks in Paris.
Security measures were stepped up outside French official buildings in various capitals including Vienna, where a international meeting on the conflict in Syria was taking place.
Governments also called for heightened vigilance from the general public.
Britain's Foreign Office updated its travel advice for France urging its citizens to exercise caution in public places while Belgium said its nationals should avoid unnecessary travel south of the border altogether.