A gunman at the wheel of a heavy truck plowed into a crowd of thousands of people celebrating Bastille Day in the French city of Nice on Thursday night.
Police say the truck was travelling at a high speed as the driver rammed it into a large crowd.
84 people were killed and 50 are in critical condition according to French President Francois Hollande.
Hollande said the attack had been carried out "to satisfy the cruelty of an individual, maybe a group'
He warned that France was "facing a struggle which will be long".
Among the dead were 10 children.
In the aftermath, their strollers and toys were seen strewn across the famous Promenade des Anglais, where people had gathered to watch a fireworks display.
— MumBaekar.. (@katamulgi) July 15, 2016
Amateur videos shows the white truck zigzagging as it deliberately smashed into the crowd.
As people started running in panic, some parents threw their kids to safety over a side fence on the road.
The attacker was a 31-year-old Frenchman born in Tunisia.
Mohamed Lahouaiej-Bouhlel opened fire before police shot him dead.
He had been known to police for petty crimes, but not to intelligence services.
No group has claimed responsibility of the attack, but President Hollande had linked it to terrorism.
Interior Minister Bernard Cazeneuve said 18 of the injured were in critical condition.
The attack, which comes eight months after DAESH gunmen and suicide bombers struck Paris, seems for now to be the work of a lone assailant.
On Friday the pavements were smeared by dried blood and debris was strewn along the Mediterranean seaside promenade.
Small areas were cordoned off at regular intervals as forensic teams assessed the scene while coroner services removed covered bodies.
A Scene of Horror
"It's a scene of horror," Member of Parliament Eric Ciotti told France Info radio, saying the truck "mowed down several hundred people."
Bystander Franck Sidoli who was visibly shocked said, "I saw people go down.
"Then the truck stopped, we were just five meters away. A woman was there, she lost her son. Her son was on the ground, bleeding," he told Reuters at the scene.
The truck, a rental vehicle according to local officials, was riddled with bullet holes from the shootout.
Robert Holloway, an AFP reporter who witnessed the incident, described scenes of "absolute chaos".
"We saw people hit and bits of debris flying around. I had to protect my face from flying debris."
State of Emergency
US President Barrack Obama offered his country's support in investigating the attack.
— The White House (@WhiteHouse) July 14, 2016
German Chancellor Angela Merkel said her country was standing with the French.
— GermanForeignOffice (@GermanyDiplo) July 15, 2016
My deepest condolences to France, those who suffered in this heinous act of terror in Nice and families of victims, Russian Prime Minister Dmitry Medvedev
Major events in France have been guarded by troops and armed police since DAESH's attacks last year.
Hollande said in a pre-dawn address that he was calling up military and police reservists to relieve forces worn out by the incident.
The country declared a state of emergency after DAESH terrorists killed 130 people in the French capital in November.
Only hours earlier Hollande had announced it would be lifted by the end of July, but he extended it by a further three months.
"France is filled with sadness by this new tragedy," he said.
A local government official said weapons and grenades were found inside the unmarked truck.
Police were trying to establish whether the driver might have had any accomplices in a city with a reputation for terrorist activism.
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.
Meanwhile, the 13th stage of global cycle race, Tour de France, got underway under tightened security after riders had observed a minute's silence in tribute to the scores of victims.
The stage is a 37.5-km time trial from Bourg-saint-Andeol to Vallon Pont-d'Arc in the Ardeche region some three hours' drive northwest of Nice.
Despite the attack, France announced that it would continue hitting DAESH positions in Syria and Iraq.