Crowds had packed out the city’s main promenade to watch a fireworks display marking Bastille Day, a major public holiday in France.
One witness reported horrific scenes with “dead bodies everywhere” and distressing video footage emerged of people fleeing the Promenade des Anglais, in the southern French city, in terror.
One eyewitnessed raised fears that the incident “had to be planned”.
They said: “I’m in Nice and cannot describe the situation – scary awful. Dead bodies everywhere people killed in front of my eyes. Had to be planned.”
A reporter for the Nice Matin newspaper who was covering the celebrations added: “There are people in blood, probably full of wounded.”
Local paper Nice Matin posted a photograph online of the white truck riddled with bullets, suggesting that police fired at the driver as it careered into the crowds. The local mayor has confirmed the death toll is in the dozens and has urged people to stay in their homes.
A witness told the paper: “We had just crossed the road and there was a huge crowd of people. We were returning and we heard ‘boom, boom, boom’ and we saw a lorry on the promenade which ploughed right through.
“My son was there right next to it. It hit where the crowd was thickest and was travelling very quickly – about 50km per hour (31mph).”
Another witness said: “He drove over everyone, everyone is bleeding. There are loads of people injured without doubt.”
It is not yet known whether or not the driver is dead or alive.
A hashtag was also started up on Twitter – #portesouvertesnice – for shellshocked witnesses and victims of the attack looking for somewhere to spend the night.
There will be fears tonight that France has been targeted by yet another horrific terrorist attack, coming after last November’s slaughter in Paris by Islamic State (ISIS) gunmen.
France is still in a State of Emergency following those attacks, when almost 150 people were murdered by Islamic State and Al-Qaeda operatives.
ISIS is well known for using vehicles in terrorist attacks, often packing them with explosives for maximum effect.