At least 30 people were killed in the southern French town of Nice when a truck ran into a crowd celebrating the Bastille Day national holiday on Thursday night, French TV channel BFM TV said.
The truck rammed into the crowd on the Promenade des Anglais seaside walk in the centre of town.
French television channel BFM TV said the local prefecture in Nice was treating the incident as an attack and was asking people to stay at home.
The reason for the crash remained unclear.
An eyewitness said there were "bodies everywhere".
Al Jazeera reporter David Coady was at the scene.
"We were just moving off. People then started running and then I heard bangs," our correspondent said.