Mournful President Hollande addresses shocked France after Nice tragedy
Although no definite terror link has been proven, experts and officials are increasingly drawing parallels between the attack in Nice - where at least 80 have died when a lorry drove into crowds - and the Paris attacks last year, claimed by the Islamic State group.
"Nothing will make us yield in our will to fight terrorism. We will further strengthen our actions in Iraq and in Syria. We will continue striking those who attack us on our own soil," he said, in reference to IS.
Hollande said several children were among the dead after the attack, which he said was of an "undeniable terrorist nature".
His response to the tragedy was to enforce even stricter security measures.
He called up reservists and extended a state of emergency, which the president had said shortly before the tragedy, would soon be lifted.
"France is horrified by what has happened, this monstrosity which is using a truck to deliberately kill dozens of people who simply came to celebrate 14 July," said Hollande.
"France was struck on its national day, a symbol of freedom."
The attack took place on a symbolic day for France - Bastille - which marks the beginning of the country's eventual transition into democracy.
"[France] will always be stronger, I promise you, than the fanatics that want to strike it".
The choice would hint that militants not only wanted to inflict damage as much damage as possible - on one of the busiest days of the year - but also hit at the very French ideas of liberty, equality and fraternity.
That would like the attack to hardline jihadi groups such as IS who has repeatedly singled out France as a prime target for its military actions against the group in Iraq and Syria.
The largest nationalist in its rank-and-file after Arabs is French.
France has been on a state of emergency since IS last struck the country in Paris on November 13.
Just hours before the attack Hollande said the state of emergency would not be renewed beyond 26 July and instead replaced by a new tougher security law.
Now the state of emergency will remain at least for another three months, he said.
He also mobilised the nation saying that although the professional armed forces were on high alert, "operational reservists" would also be called up to boost the ranks of police and gendarmes.
These include French citizens with or without military experience as well as former soldiers.
Agencies contributed to this story.