In Nice, at about 10.30 pm yesterday local time a truck slammed into a crowd of people celebrating Bastille Day and barrelled about 2 km.
Authorities said they found identity papers belonging to a 31-year-old French-Tunisian citizen in the 19-tonne truck, and that the driver had fired a gun several times before police shot him dead.
The attack was of an "undeniable terrorist nature," a sombre Hollande said in a televised national address, confirming that several children were among the dead, The Local informs.
"France was struck on its national day ... the symbol of freedom," said Hollande.
Bastille Day is a celebration of everything France holds dear – its secular republic and the values of "Liberte, Egalite, Fraternite" (Freedom, Equality, Fraternity).
Interior ministry spokesman Pierre-Henry Brandet said 84 people were killed and scores injured, including 18 in "critical condition".
The truck was riddled with bullet holes and badly damaged, with burst tyres.
A source close to the investigation said an "inactive" grenade was found inside the truck, as well as "several fake rifles".
The attack was the third major strike against France in less than 18 months and prosecutors said anti-terrorist investigators would handle the probe.
It comes eight months after Islamic State attacks on Paris nightspots left 130 people dead, dealing a hard blow to tourism in one of the world's top destinations.
US President Barack Obama condemned "what appears to be a horrific terrorist attack", although no group had yet claimed responsibility.
Hollande announced he would extend France's state of emergency for three months in the wake of this latest attack and "step up" the government's action against jihadists in Syria and Iraq.
"We will continue striking those who attack us on our own soil," he said, in reference to the Islamic State group.
He also called up army reservists to bolster security services that are stretched to the limit. France has been under a state of emergency ever since the November 13 Paris carnage, which came after 17 were killed in another attack in January at various sites including the offices of the Charlie Hebdo magazine and a Jewish supermarket.
Isis has repeatedly singled out France as a prime target for its military action against the group in Iraq and Syria, and hundreds of jihadists have left France to go and fight in its ranks.
Over the past week, France had been breathing a sigh of relief after successfully hosting the month-long Euro 2016 football championship, which passed off without incident despite fears of attacks.
The tournament brought an all-too-brief burst of joy to a gloomy France, bogged down after the two attacks in 2015, violent anti-government protests, strikes and floods.