A man drove a lorry into a crowd who had gathered on the seafront promenade in the town.
There were also reports of gunfire.
Police say more than 100 people have been injured which is a popular tourist destination for people from Ireland.
The promenade where the lorry drove into crowds in Nice was “littered with dead bodies”, an eyewitness told ITV News.
Alex Maule told ITV News how she hid in a casino for more than an hour as the incident happened during France’s Bastille Day celebrations.
She said: “People were screaming and shouting. There were running all over the place.
“They were shouting ‘there’s a lorry, there’s a lorry.’ It was chaos.
“They started bringing people inside one by one in various states of injury – some had head wounds, some were bleeding from their arms or legs.
“They were bringing unconscious children in.”
When Ms Maule was finally able to go back outside she was met with more horrific scenes.
“I walked on to the promenade and it was littered with dead bodies covered in table cloths,” she said.
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