Security forces and ambulances raced to the scene, and authorities from the local Alpes-Maritimes prefecture urged residents to stay indoors.
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.