France mourns 84 dead in Nice truck attack

President Francois Hollande said the country would observe three days of mourning as he warned the death toll could rise further, with more than 50 people fighting for their lives following the attack in the Cote d'Azur resort city on Thursday night.

Prime Minister Manuel Valls said attacker Mohamed Lahouaiej-Bouhlel probably had links to radical Islam, but Interior Minister Bernard Cazeneuve cautioned it was too early to make the connection.

Anti-terrorism prosecutor Francois Molins said the 31-year-old Tunisian was 'completely unknown' to the intelligence services but that the assault was 'exactly in line with' calls from jihadist groups to kill.

The attack left a scene of carnage on Nice's picturesque Promenade des Anglais, with mangled bodies strewn over the palm-fringed walkway. At least 10 children and teenagers were among the dead.

AFP reporter Robert Holloway witnessed the white truck driving at speed into the crowd, causing 'absolute chaos'.

"It was hurtling towards us and we had just enough time to yell at each other 'get out of the way!'," he said.

Dramatic video footage showed police surrounding the heavily damaged truck and firing through the windscreen to kill the attacker.

The massacre again prompted questions as to why France is a persistent target for attacks and what can be done to prevent such unsophisticated assaults.

Investigators were building up a picture of Lahouaiej-Bouhlel; a man with a record of petty crime, but no known connection to terrorist groups.

His father said he had suffered from depression and had 'no links' to religion.

"From 2002 to 2004, he had problems that caused a nervous breakdown. He would become angry and he shouted... he would break anything he saw in front of him," Mohamed Mondher Lahouaiej-Bouhlel said outside his home in Msaken, eastern Tunisia.

"We are also shocked," he said, adding that he had not seen his son since he left for France but was not entirely sure when this was.

Neighbours described the attacker, who lived in a modest district of Nice and worked as a delivery man, as a loner who never responded to their greetings.

He and his wife had three children, but she had demanded a divorce after a 'violent argument', one neighbour said.

His wife was arrested on Friday and taken for questioning, a police source said.