Dozens of people have been killed, including children, after a lorry ploughed into a large crowd watching a fireworks display marking the end of the French national holiday for Bastille Day.
The driver also fired shots, before being killed by police. This is what we know so far about what happened.
Who is the attacker?
The driver of the lorry has been identified by officials as Mohamed Lahouaiej Bouhlel, a 31-year-old man of Franco-Tunisian origin.
Paris prosecutor Francois Molins says Bouhlel was divorced with three children. His ex-wife was taken into custody on Friday morning. A flat he lived in near Nice train station was searched by police on Friday morning.
Mr Molins said Bouhlel was “totally unknown” to security services, and the investigation is still investigating whether Bouhlel acted alone.
He is said to have hired the lorry from a rental company in Saint-Laurent-du-Var, a town to the west of Nice, on 11 July, and had been due to return it on 13 July.
Police said Bouhlel was in possession of an automatic pistol, bullets, a fake automatic pistol and two replica assault rifles (a Kalashnikov and an M16), an empty grenade. Also in the lorry with him were a driving licence and a bank card.
Who were the victims?
At least 84 people were killed, including 10 children and teenagers.
In the hours after the attacks, worried relatives posted images on social media of the missing.
Among the dead was Fatima Charrihi, whose son said she was the first to die.
Another victim, according to reports, was the assistant head of the Nice border police, Jean-Marc Leclerc.
An American boy, Brodie Copeland, and his father, Sean, were also killed. They had been on holiday in Nice.
Three people on a school trip from Germany were unaccounted for.
Who was behind the attack?
French security officials are still assessing whether the driver of a truck was working alone or in a group.
No group has said it carried out the attack, but officials said it bore the hallmark of a terrorist organisation.
President Hollande said earlier on Friday that it was “an attack whose terrorist nature cannot be denied”.
Mr Molins said the attack was “in line with the constant calls to kill” from militant Islamist groups, and the investigation would be seek to find out whether Bouhlel had ties to Islamist militants.
Anti-terrorist prosecutors in Paris have launched an inquiry for murder and attempted murder as part of an organised terrorist strike.
Earlier this week, France’s DGSI internal security organisation warned of the danger of further attacks from Islamist militants with “booby-trapped vehicles and bombs”.
The so-called Islamic State has targeted France on several occasions since January 2015.
Only hours before the Nice attack, President Hollande had announced that France’s state of emergency would be removed later this month. He later announced it was being extended.
What happened on the promenade?
The terror began a little after 22:30 (20:30 GMT) on Thursday, shortly after thousands of people had watched a firework display on the seafront in Nice, at the end of a day marking France’s national holiday, Bastille Day.
There had been a mood of celebration and the crowd had enjoyed an Air Force display. Families strolled along the city’s renowned Promenade des Anglais.
A large white lorry was seen driving erratically a couple of streets away from the seafront promenade. “He was speeding up, braking, speeding up again and braking again. We thought it was weird,” said Laicia Baroi. She described how the lorry then turned on to the promenade heading south-west towards the airport.
But it was not for another half hour before the attack began. A German journalist saw events unfold from a hotel balcony, as the lorry doubled back from the direction of the airport, breaching the barriers erected on the promenade opposite the Lenval children’s hospital.
When two police officers opened fire on the lorry, the driver accelerated and careered at full speed towards the crowd.
The vehicle mounted the kerb then went back on the road, zigzagging for up to 2km (1.25 miles), as the driver deliberately drove into people.
A local MP spoke of hundreds of people being run over. Others scrambled to safety, on to the beach or into nearby hotels.
“I was opposite the Palais de la Mediterranee [hotel] when I saw a lorry at high speed running over people. I saw it with my own eyes, people tried to stop it,” said one witness.
Police finally managed to bring the lorry to a halt near the luxury hotel.
The French Prosecutor Francois Molins says the driver fired repeatedly on three policemen, who returned fire and pursued him for hundreds of meters.
Mobile phone footage appeared to show the moment the driver was shot.
Images from the scene showed the windscreen and front of the lorry raked with bullets. Interior ministry officials later confirmed that the attacker had been “neutralised”.
How have the authorities reacted?
It soon became clear that many people had died, although the full scale of the disaster was unclear. The dead and injured were taken to the local Centre Hospitalier Universitaire de Nice.
The hospital launched its crisis mode for exceptional health situations and put out an emergency number for families: 00 33 (0)4 93 72 22 22.
In the area around Nice, the anti-terror alert was raised to its highest level.
President Francois Hollande was flown back to Paris from a visit to Avignon, joining Prime Minister Manuel Valls in a crisis room. Mr Valls declared three days of mourning starting on Saturday.
The pair then travelled to Nice, where the interior and health ministers were already involved in crisis meetings with local officials.