A lorry has struck a crowd during Bastille Day celebrations in the southern French city of Nice, killing about 60 people, officials are quoted as saying by local media.
The incident took place on the famous Promenade des Anglais during a firework display.
One image on Twitter showed about a dozen people lying on the street.
The local prefecture has urged people in the area to remain indoors, calling the incident "an attack".
Nice Mayor Christian Estrosi said that "a lorry driver appears to have killed dozens of people".
Sebastien Humbert, the prefect of the Alpes-Maritimes region, told France's BFM TV that the lorry driver had been shot dead.
French media later quoted Nice prosecutors as saying that about 60 people were dead.
Prosecutor Jean-Michel Pretre said the lorry drove two kilometres (1.2 miles) through a large crowd, the AFP news agency reports.
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The mayor and police urged residents to stay indoors.
Image copyright TwitterFrench President Francois Hollande was returning back to Paris for crisis talks in the capital, his office said. Mr Hollande was in the southern city of Avignon on a private visit.
Some reports spoke of shots being exchanged between police and the occupants of the lorry but these have not been confirmed.
Social media video showed people running through the streets in panic following the incident.
A journalist with the Nice Matin newspaper reported from the scene that there was "a lot of blood and without doubt many injured".
Image copyright AFPAn AFP 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.
One eyewitness told BFM TV: "Everyone was calling run, run, run there's an attack run, run, run. We heard some shots. We thought they were fireworks because it's the 14th of July.
"There was great panic. We were running too because we didn't want to stick around and we went into a hotel to get to safety. "
Another witness, Roy Calley told the BBC that there were "thousands of people on the promenade" when the incident happened.
US President Barack Obama had been briefed about the situation in Nice "and his national security team will update him, as appropriate", National Security Council spokesman Ned Price said.
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