More than 70 people were killed when a truck plowed into a crowd in the southern French city of Nice during Bastille Day celebrations, broadcaster BFMTV reported.
The president of the region, Christian Estrosi, said on Twitter that there were 70 victims and called it the worst incident in Nice history.
Sebastien Humbert, the sub-prefect of the Alpes-Maritimes, earlier told BFMTV broadcaster that there were many dead and injured, attributing the heavy toll to the truck having driven along the main promenade.
Interior Ministry spokesman Pierre-Henry Brandet said the driver of the truck had been "neutralized" by police. French media said that the truck had traversed 2 kilometres of the promenade where people had gathered to celebrate.
There were shots fired during the attack, but it was not immediately clear if the shots were fired by people in the vehicle or by police. An operation was underway around midnight.
City officials advised people to stay away from the center of the city and to remain inside. The police asked residents to remain calm and to not propagate rumours.
Local newspaper Nice Matin reported that panic broke out on the promenade, and videos showed people running along the streets of Nice. Eyewitnesses told broadcasters that they saw the truck leave a trail of victims.
France's Interior Ministry said that it had convened a crisis meeting and minister Bernard Cazeneuve was en route to Nice. The Elysee Palace said President Francois Hollande was travelling to take part in the crisis meeting in Paris.
France is still under a state of emergency that was put into place after a series of terrorist attacks in Paris last November.