At least 84 people have been killed and dozens injured after a truck careered along Nice's packed waterfront, ploughing into hundreds of people.
At least 10 children are among the dead after what French President Francois Hollande described as a monstrous "terrorist" act.
Prosecutors say 202 people were injured, including 52 who are now in 'critical care'. 25 of those are reported to be in intensive care.
Three days of mourning have been declared in France following the attack.
The man who drove the lorry into the crowd - who were celebrating Bastille Day - has been named as 31-year-old Mohamed Lahouaiej Bouhlel.
Paris prosecutor Francois Molins said the French-Tunisian national had a history of petty crimes prior to Thursday's attack.
He was known for domestic violence, threats and theft.
Bouhlel had also been given a six-month suspended jail sentence in March for a road rage incident in January.
French Justice Minister Jean-Jacques Urvoas described the road rage incident as "an altercation between him and another driver" adding that he had "hurled a wooden pallet at the man".
But Mr Molins said he was "totally unknown" to the French intelligence services and had "never been subject to any sign of radicalisation".
Earlier, authorities said they had found "grenades and larger weapons" in the truck, which had picked up speed and travelled along the famous Promenade des Anglais for more than a mile.
However, Mr Molins said later they had found one pistol and a number of fake weapons.
Bouhlel was born in Tunisia and lived in Nice, where he worked as a delivery driver.
He had held a French residence permit for 10 years but had not gained French nationality, according to Tunisian sources.
On Friday, police raided a 12th floor apartment occupied by his estranged wife, who was led away by the authorities.
She has been detained and remains in custody.
Taoiseach Enda Kenny said this morning that an Irish person is believed to have been critically injured in the attack.
Mr Kenny told reporters that there was worry about one particular citizen in the city:
The Department of Foreign Affairs confirmed it was "urgently" following up on "particular concerns" for the person's welfare.
"We will provide an update on the situation in due course," it said, adding that embassy staff were in ongoing contact with French authorities.
Video footage from the attack showed police trying to stop the lorry before it picks up speed, slamming into revellers gathered along the city's famous waterside Promenade des Anglais for a Bastille Day fireworks display.
The driver zig-zagged along the promenade for some two kilometres (1.3 miles), mowing people down along the way, according to Nice prosecutor Jean-Michel Pretre.
Several officials say he then emerged from the vehicle and started shooting, sending hundreds of panic-stricken men, women and children running in terror.
"We almost died. It was like hallucinating... (the lorry) zig-zagged," an eyewitness told i-Tele.
"You had no idea where it was going. My wife... a metre away ... she was dead.
"The lorry ripped through everything. Poles, trees, we have never seen anything like it in our lives. Some people were hanging on the door trying to stop it."
Mouvement de panique !! #Nice pic.twitter.com/RgXar3fWZs— HARP DETECTIVES (@harp_detectives) July 14, 2016
Graphic footage from the scene shows an endless succession of bodies left strewn along the city's landmark promenade in the lorry's wake.
Screams and cries can be heard as dazed witnesses lean over the bodies, looking for survivors.
Items of clothing and shoes are scattered across the blood-stained pavement.
French officials say the driver was shot dead by police at the scene, with images showing the windscreen of the 25-tonne lorry riddled with bullet holes.
In a live TV address, the French president said there is "no denying the terrorist nature of this attack".
"I express in the name of the nation, our tears, solidarity, with the victims and the families," Mr Hollande said.
He pledged to strengthen France's role in Syria and Iraq and extended the nation's state of emergency - imposed after November's attacks in Paris - for a further three months.
La France est éplorée, affligée, mais elle est forte et le sera toujours plus que les fanatiques qui veulent aujourd’hui la frapper. #Nice— François Hollande (@fhollande) July 15, 2016
Nice's famous luxury Negresco hotel, meanwhile, has been turned into a makeshift ward, with dozens of frightened and injured victims gathered in the marble lobby.
The hashtag #RecherchesNice has been started for people still looking for loved ones.
"This is the worst disaster in the history of Nice as more than 70 people have already lost their lives, killed by the driver as he fired at the crowds ahead of him," Mr Estrosi said in a statement.
"We are terrified and we want to express our sincere condolences to the families involved.
"Today, we must act. We are currently in touch with the State authorities who are dealing with this crisis."
The Paris prosecutor's has office opened an investigation for "murder, attempted murder in an organised group linked to a terrorist enterprise". The probe is being handled by France's intelligence agency and judicial police.
Authorities in Nice told local residents to stay in their homes as they conducted further operations, though there has been no sign of any other threats.
Minister for Foreign Charlie Flanagan last night extended condolences to the people of France.
"This attack on people as they celebrated Bastille Day with friends and family on a fine summer's evening is particularly horrendous, and my thoughts and sympathies are with the relatives of the dead and injured," he said.
Mr Flanagan that staff in his department were working with Ireland's ambassador in France, Geraldine Byrne Nason, to establish whether any Irish citizens were caught up in the tragic events.
Irish citizens in Nice who need to contact the embassy can do so on 01 441 76700. Concerned families here in Ireland can also call consular staff on 01 408 2000.
Irish citizens in #Nice who need to contact the Embassy can do so at 0144176700. Stay safe and follow instructions of local authorities.
— Irish Embassy Paris (@IrlEmbParis) July 14, 2016
Mr Kenny said he was "deeply shocked and saddened" at the attack.
"Once again innocent people have been targeted at an occasion of joy and celebration," he said.
"French people have suffered appallingly and have again been the victim of cynical, and wanton violence. We cannot and will not yield to this malevolence.
"On behalf of myself, the government and the Irish people I want to express our deepest sympathies and our solidarity to President Hollande and to all of France."
The attack comes days after France breathed a sigh of relief after hosting the Euros 2016 without incident.
The tournament took place under heavy security following the Paris attack that left 130 dead.
