Supporters of Islamic State celebrated on social media an attack in France by a truck that ran into crowds watching a fireworks display on the Bastille Day national holiday, killing at least 80 people and injuring over 100.
"The number of those killed had reached 62 french crusaders and sinful infidels in Nice, France.. God is great, God is great," read one tweet as details of the terror attack were emerging.
“Oh France, you and the all Europe will never be secure until we will live secure on every inch in the land of the Caliphate,” wrote another user.
“This is the beginning of the attack to take the holy revenge for the killing of Abu Omar Shishani, may Allah accept him,” another said on an Isis forum.
Police source close to the investigation told global news agency Reuters that the gunmen is believed to be a 31-year-old Franco-Tunisian born in Tunisia.
The man was not on the watch list of French intelligence services, but was known to police in connection with common law crimes such as theft and violence, the source said.
In a pre-dawn address to the nation, French President Francois Hollande said Thursday's lethal attack on the southern city of Nice was clearly a terrorist assault and that the state of emergency imposed since a previous attack on Paris last November would be extended for another three months.
Hollande said at least 80 people were killed in an attack in which the assailant drove a big truck at high speed into dense crowds who were watching a fireworks display on the country's national Bastille Day holiday.
"There's no denying the terrorist nature of this attack of yet again the most extreme form of violence," the French leader said in a national television address at 4 a.m. (0200 GMT), about five or six hours after the carnage in Nice.
President Francois Hollande called up military and police reservists to relieve forces worn out by an eight-month state of emergency begun after the Islamic State militant group killed 130 people in Paris. The state of emergency has been extended.
"France is filled with sadness by this new tragedy," Hollande said, noting several children were among the dead in what he said he had no doubt was an act of terrorism.
He called the carnage, which came as France celebrated the anniversary of the 1789 revolutionary storming of the Bastille, an attack on liberty by fanatics who despised human rights.
France would, nonetheless, continue military operations in Syria and Iraq.
The driver drove at high speed for over 100 metres (yards) along the famed Promenade des Anglais seafront in the French Riviera city of Nice before hitting the mass of spectators. The driver was shot dead, sub-prefect Sebastien Humbert told France Infos radio.
The driver was not yet identified. Residents of the Mediterranean city, which is close to the Italian border, were advised to stay indoors. There was no sign of any other attack.