At least 84 people have died, including children, after a lorry slammed through a crowd celebrating Bastille Day in the southern French city of Nice, officials say.
The driver of the truck, which was loaded with weapons, has been killed by French police.
President Hollande said the attack was of a 'terrorist nature'.
He said: “France is badly hit.
"We need to do everything we can to fight against. All of France is under the threat of Islamic terrorism."
Investigations are taking place into whether the driver was acting alone or with a group of people. No terror group has claimed responsibility yet but the attack is being handled by anti-terror investigators.
The state of emergency in France following the Paris attacks last November, killing 130 and leaving hundreds wounded, has been extended.
Social media posts show people lying injured on the ground and running through the streets in panic, taking shelter in homes and businesses.
One eyewitness told BFM TV: "Everyone was calling run, run, run there's an attack run, run, run. We heard some shots. We thought they were fireworks because it's the 14th of July.
"There was great panic. We were running too because we didn't want to stick around and we went into a hotel to get to safety. "
A journalist with the Nice Matin newspaper reported from the scene that there “was a lot of blood and without doubt many injured.”
The attacks took place late last night after a firewok display along the famous Promenade des Anglais, in Bastille Day celebrations – France's national day.
According to news agency afp, Prosecutor Jean - Michel Petre said the lorry drove 1.2km through the large crowd. Witnesses also added that the driver was “zigzagging” so he could hit as many people as possible.
US President Barack Obama condemned "in the strongest terms" what he said appeared to be "a horrific terrorist attack in Nice", the White House said.
National Security Council spokesman Ned Price said that the president had been briefed about the situation "and his national security team will update him, as appropriate".
British Foreign Secretary Boris Johnson said he was "shocked and saddened by the appalling events in Nice, and the terrible loss of life".
European Council President Donald Tusk condemned the attack, saying "it's a tragic paradox that the subject of this attack were people celebrating liberty, equality and fraternity".
**Nice hospitals have launched an appeal for blood donations in the wake of the attack. Spread the word, do what you can. **
French police have urged people NOT to share graphic pictures or videos of the horrific events out of respect for victims families.
**We'll have more on this story as it happens. **