The French leader said the atrocities, which rocked the French capital on Friday, had been organized and planned from abroad, Reuters reported.
He said the attacks on a stadium, concert hall and Paris cafe diners were "committed by a terrorist army, the Islamic State (ISIL) group, against France, against the values that we defend everywhere in the world, against what we are: A free country that means something to the whole planet."
He added France "will be merciless toward the barbarians of Islamic State (ISIL) group."
France "will act by all means anywhere, inside or outside the country."
Calling the attack "cowardly", Hollande said every effort would be made to fight “the terrorist menace,” and said he would be addressing the French parliament on Monday.
“I pay homage to the country’s defenders who fought the terrorists yesterday. Everyone has given their upmost and will be putting in their best efforts in the day to come.”
“In this most serious and uncertain time, I call for unity and courage," he added.
“Even if France is wounded, she will rise.”
President Hollande confirmed that the death toll now stands at 127 and declared there would be three days of national mourning.