Sixty people have been killed while several were injured after a van rammed into a crowd marking Bastille Day in the French Riviera resort on Thursday.
Security forces and ambulances raced to the scene, and authorities from the local Alpes-Maritimes prefecture urged residents to stay indoors.
One eyewitness reported seeing the driver slam the lorry into revellers celebrating Bastille Day on the city’s famous Promenade des Anglais.
He then emerged from the vehicle and started shooting, local resident Wassim Bouhlel told news agency AP.
“There was carnage on the road, dead bodies everywhere,” he said.
The BBC reports that shots were exchanged between the police and the occupants of the lorry.
A journalist with the Nice Matin newspaper reported from the scene that there was “a lot of blood and without doubt many injured”.
An Agence France-Presse reporter said the incident took place as the firework display was ending, adding: “We saw people hit and bits of debris flying around.”
Another image on Twitter showed a white lorry stopped in the middle of the promenade with damage to its front, and four police officers observing it while taking cover behind a palm tree.
An AFP reporter described seeing a white van driving at high speed onto the famed Promenade des Anglais as people were leaving after the annual Bastille Day celebration display.
“We saw people hit and bits of debris flying around,” he said, adding that the incident took place near the city’s famed Hotel Negresco.
Terrified pedestrians screamed as they fled the area. “It was absolute chaos,” he said.
A huge security cordon was established closing off the central Place Massena by 2330 GMT, another AFP correspondent said.