Witnesses of the attack in Nice described seeing people run through the streets as they heard gunshots.
At least 80 people, including several children, are dead after the suspected terror attack on Bastille Day celebrations.
A further 18 people are in a critical condition as a lorry hit crowds who had gathered to celebrate the French national day in the Mediterranean city on Thursday night.
Wassim Bouhel told the French TV channel iTele that the lorry zigzagged across the road.
He said: “We almost died. It was like hallucinating ... (the lorry) zigzagged - you had no idea where it was going. My wife ... a metre away ... she was dead.
“The lorry ripped through everything ... poles, trees.
“We have never seen anything like it. Some people were hanging on the door and tried to stop it.”
Witness Lucy Nesbitt-Comaskey told Sky News that the noise of gunfire “sounded like Beirut”.
She said: “I said to my friend ‘This doesn’t sound like fireworks, it sounds like Beirut when it’s under fire’.
“All of a sudden people were screaming in the streets and running into all the restaurants.
“All the restaurants were open and people were coming.
“We were just sitting there and everyone came into our restaurant and the owners were saying ‘Please don’t go anywhere, come in, come in’.
Ms Nesbitt-Comaskey said she and her friend were planning to go to the fireworks but stopped to find a toilet and were only a block away when the attack happened.
We have never seen anything like it. Some people were hanging on the door and tried to stop it.Eyewitness in Nice
Speaking about what she witnessed, she said: “It was shocking, it was devastating and I cannot believe that I have come over here for a few days and I have got mixed up in something so tragic.
“It was just awful.”
Briton Will Shore was in a nearby bar when he heard gunfire and said his initial reaction was to run towards the city centre to see what was happening.
He told the BBC: “I kind of ran towards the centre of Nice where there was a rather large jazz festival, and something was going on. I immediately found that military and the local police were just ushering absolutely everybody out of the area.
“It was quite chaotic really. There was a lot of people screaming, running around and people were kind of being pushed over, I think, from people just being so frightened about what was going on, especially after hearing the gunshots.
“I had to help a couple of people up who were in distress on the floor because everyone was in such a panic.”
He added: “You could genuinely see the fear and panic in people’s faces when they were running away. It was a mass amount of people running away.“
Kevin Harris watched the attack unfold from his balcony.
He told the BBC: “I saw what appeared to be bodies in the lying in the road.
“I spoke to my neighbour, who said the lorry had ploughed through the people. It’s a terrible scene.”
Celia Delcourt, 20, from Nice, was enjoying the Bastille Day celebrations with friends when the attack happened.
“When the fireworks ended, we went on to the Promenade des Anglais and we started walking and we heard gunshots and we started running from on the other side,” she told ITV’s Good Morning Britain programme.
“It was dark. We didn’t know what was happening. We thought it was fireworks from another place. We just started running because everybody was running, without knowing what was happening.
“We never thought it will happen in our city, it’s crazy. It feels like it’s a part of our day since it happened in Paris but we never thought it would happen here.”
Ms Delcourt added: “Everybody is shocked we have never been in a situation like this so it is unbelievable. It’s shocking.”
Lawyer Harjit Sarang and her children were among those caught up in the terror.
The Londoner tweeted: “Running through crowds in Nice with kids and terrified. Never taking kids to a public event again. Finally back to hotel. Hate this!
“F****** scariest thing ever running through crowds with boys. Got back to hotel and couldn’t get in for people seeking refuge!
“Can’t stop shaking. Hate that my boys had to experience this. Why did I take them. Why did they do this and why the f*** is this happening!”
Roy Calley is a BBC journalist who lives just a few metres from the current police cordon in Nice.
He had been watching the firework display when the attack started: “I heard what at first I thought was a bit of an explosion, though maybe that was my imagination.
“Almost instantaneously people started to scream and run in all directions and the police, who were not on the promenade, who were redirecting the traffic in the city centre, then started running in the opposite direction towards me and made it a very firm point that we were to leave as quickly as possible.”
He said he then heard what he thought were firecrackers but now believes were gunshots.
“It was a case of panic for a good 10 to 15 minutes following,” he said.
Mr Calley was on the Promenade des Anglais on Friday morning and described the atmosphere as “eerie”.