At least 84 were killed in the southern French city when a truck was driven through a crowd that had gathered for Bastille Day celebrations.
Some people jumped off the promenade seeking safety, he says. According to a government official, police found guns and explosives in the back of the truck, NPR's Eleanor Beardsley reports.
"This is war", Trump said after the apparent terrorist attack.
France declares three days of national mourning after attack. Fifty people remain in critical condition, many of whom are children, according to Hollande.
"I heard like two pops". After that there was no conscious thought, my body took over, time slowed down and I ran and thank God I got out of the way. After Charlie Hebdo, we forgot, and then we had the Bataclan and then we forgot, and we had Brussels, and after Brussels we forgot, and now we have Nice.
Kosachev noted that people grieve together "with those who was killed and injured in Nice, no matter the nationality and citizenship".
"It zigzagged - you had no idea where it was going". It passed two metres away from us. The lorry ripped through everything... poles, trees. "Bodies everywhere", he said. The driver was eventually shot dead by police.
Once again France is waking up to mass murder on its streets and a profound sense of shock.
A truck smashed through bystanders near the iconic Promenade des Anglais in Nice. There is an eerie sense of unease and disbelief that jars with the attractive beachfront surroundings.
The driver of the large white commercial truck killed scores of people as he paved his way into a pedestrian-only area as crowds watched Bastille Day fireworks. It has seen dozens of its Muslim residents travel to Syria to fight, a path taken by previous Islamic State attackers in Europe. Pictures show the truck's windscreen riddled with bullets.
Stephanie Simpson, the communications director for the Lenval foundation hospital, tells The Associated Press that injuries included fractures and head injuries and that the victims were aged 18 or under.
Christian Estrosi, president of the Provence-Alpes-Cote d'Azur region, called the attack "the worst tragedy in the history of Nice" because of the death toll.
France is also Islamic State's biggest source for European recruits.
No terrorist organisation has yet claimed responsibility for the attack. At home, France has become the number one target of opportunity for IS and its supporters, unperturbed by the national state of emergency that has just been extended.
No group has so far said it was behind the attack; prosecutors said the inquiry would be handled by anti-terror investigators. The attacker is identified as a Tunisian-born Frenchman. "France as a whole is under threat of Islamic terrorism".
Hollande continued, "I have chose to first maintain a high level of police forces, with 10,000 military staff, as well as our police forces".
Word quickly spread to those in neighbouring streets, flooding the city with panic, confusion and fear.
"But France is a great country and a great democracy that will not allow itself to be destabilised", he said.
Police investigate the scene after a truck plowed through Bastille Day revelers in the French resort city of Nice, France, Thursday, July 14, 2016.