At least 84 people, including children, were killed and up to 100 people were injured when a heavy truck ploughed into a crowd watching the Bastille Day fireworks in the southern French Riviera city of Nice late on Thursday.
The incident took place on the famous Promenade des Anglais. The police shot and killed the driver. The truck was loaded with weapons and grenades.
The police found identity papers belonging to a 31-year-old French-Tunisian inside the truck. "The identification of the truck driver is still under way. The identity papers indicate the man is a resident of Nice," a police official told AFP.
The authorities have advised the residents of Nice to stay indoors.
President Francois Hollande addressed the nation and said the attack was clearly of a "terrorist nature". He extended a state of emergency across France by three months, which has been in place since the deadly Paris attack in last November.
"France is filled with sadness by this new tragedy," Hollande said. He said that several children were among the dead and he is sure that this was an act of terrorism.
In reference to the Islamic State in Iraq and the Levant (ISIL, also known as ISIS) armed group, Hollande said: "Nothing will make us yield in our will to fight terrorism. We will further strengthen our actions in Iraq and in Syria. We will continue striking those who attack us on our own soil."
However, there was no immediate claim of responsibility for the attack on Thursday. But Nada Bakos, a former CIA analyst, told Al Jazeera that the attack "looks very similar" to some of the attacks which were carried out by Al-Qaeda and ISIL.