Speaking at the site of the Bataclan concert hall in central Paris -- the site of the most severe attack where at least 112 people died after gunmen stormed the venue, Hollande said: "France will not let itself be overwhelmed or frightened even if today we are overcome with sadness and emotion."
The President, speaking on Saturday, said ISIS was responsible for the attacks.
"It's an act of war, committed by a terrorist army Daesh (ISIS), an army of Jihadists, against France," Hollande said.
Earlier, the President described the carnage as an "abomination." He said the attackers were intent on killing as many people as possible.
"I want to state my solidarity and compassion for all the families who are impacted and I want to point out once again how efficient the security forces were in carrying out their duty in the extremely difficult conditions," Hollande said late Friday night.
Witnesses described how gunmen in black started shooting and after wounded people fell to the floor, they were shot again, execution-style.
Security forces later stormed the building, killing four attackers -- including three who were wearing explosives belts, Paris police spokesman Michel Cadot told France Info radio.
After praising the work of the firemen, paramedics and doctors who dealt with the extremely disturbing scenes across Paris, Hollande on Friday vowed a tough response.
"We will lead the fight and we will be ruthless, and we had to be here among the people who were subject to these atrocities because when the terrorists are capable of doing such acts they must know that they will face a France very determined -- a France united."