French President François Hollande has confirmed the killing of many, including children, during a press conference following an attack in Nice.
He also says it was clearly a terrorist attack.
"France has been struck on the day of its national holiday - the symbol of liberty. We show our solidarity towards the victims and their families. All means are being deployed to help the wounded," he says.
"France as a whole is under the threat of Islamic terrorism. We have to demonstrate absolute vigilance and show determination that is unfailing."
Mr Hollande announced that a state of emergency, which was supposed to end on 26 July, will be extended by three months.
He said the government had called up operational reserves to help support French armed forces and police, especially in controlling borders.
"Nothing will lead us to give in to our will to fight against terrorism," he said.
"We are going to strengthen our efforts in Syria and Iraq against those who are attacking us on our very soil."
Mr Hollande said he would go to Nice on Friday to support the city.
"France is horrified by what has taken place, this monstrosity of using a truck to deliberately kill dozens of people who had come to celebrate 14 July.
"France is afflicted but I assure you we are strong and we will always be stronger than the fanatics that want to strike us today."