“France has been struck on the day of her national holiday — July 14, Bastille day — the symbol of liberty, because human rights are denied by fanatics and France is clearly their target,” François Hollande, the French president, said in an address to the nation.
“The terrorist nature of this attack cannot be denied,” he added, saying that all of France was “under the threat of Islamist terrorism”.
The dead and injured were scattered along 2km of the Promenade des Anglais seafront, local officials said, estimating that another 100 were injured at the Bastille day celebrations. Mr Hollande said the dead included several children and as many as 20 of the injured were in critical condition.
Mr Hollande extended for three months the state of emergency that was imposed after November’s Isis-inspired attacks on Paris and had been due to end on July 26.
France would step up its strikes in Syria and Iraq, he said. “We will continue to hit those who threaten us.”
The truck was loaded with grenades and guns, according to Christian Estrosi, president of the Provence-Alpes-Côte d’Azur region and mayor of Nice until last month.
“The driver was behaving in a manner that by all evidence was structured and premeditated,” he told a press conference. He said the attack happened at 10.30pm local time.
“This is the worst tragedy in the history of Nice,” Mr Estrosi wrote on Twitter.
The French interior ministry confirmed that the driver had been shot and killed but denied early reports of a hostage crisis.
The driver had driven at high speed into the mass of spectators on the Mediterranean city’s seaside walkway, Sebastien Humbert, a regional sub-prefect, told France Info radio.
Mr Hollande did not reveal the identity of the driver, saying investigators needed to establish whether he had any accomplices. The president, who had been in Avignon in the south of France at the time of the attack, was flown to Paris.
Mr Hollande said he would fly to Nice after a meeting with his defence council on Friday. Bernard Cazeneuve, interior minister, arrived in Nice on Thursday night to help co-ordinate the response.
In a statement, US President Barack Obama condemned “what appears to be a horrific terrorist attack” and said he had directed his team to offer French officials “any assistance that they may need to investigate this attack and bring those responsible to justice”.
Hillary Clinton, the Democratic presidential candidate, said: “We will never allow terrorists to undermine the egalitarian and democratic values that underpin our very way of life. This cowardly attack only strengthens our commitment to our alliance and to defeating terrorism around the world.”
Donald Trump, the Republican presidential candidate, postponed the planned announcement of his vice-presidential running mate at an event on Friday in Manhattan “in light of the horrible attack in Nice, France”.
In response to the news from Nice, he expressed his prayers and condolences and tweeted: “Another horrific attack, this time in Nice, France. Many dead and injured. When will we learn? It is only getting worse.”
The attack came eight months after Isis militants killed 130 people in Paris, the bloodiest in a number of attacks in France and Belgium in the past two years. On Sunday, France had breathed a sigh of relief as the month-long Euro 2016 football tournament it hosted ended without an attack.
Hours before the attack Mr Hollande had said the state of emergency imposed in November would not be extended beyond July 26.
“We can’t extend the state of emergency indefinitely, it would make no sense. That would mean we’re no longer a republic with the rule of law applied in all circumstances,” he had told journalists in a traditional Bastille day interview.
Mr Estrosi had warned in the past of the risk of Islamist attacks in the region, following the Isis bloodshed in Paris and Brussels.
Officials were cautious about accounts from bystanders that the lone man driving the truck had also opened fire. Police called on people not to propagate rumours that were hampering their work.
One woman told France Info that she and others had fled in terror: “The lorry came zigzagging along the street. We ran into a hotel and hid in the toilets with lots of people.”
The Negresco hotel on the Promenade was turned into a makeshift hospital for the wounded.
Another woman told the station she was sheltering in a restaurant on the promenade with about 200 people, where things had calmed down about two hours after the incident.
Nice Matin, a regional newspaper, published a photograph of a damaged, long-distance delivery truck, which it said was riddled with bullets, and images of emergency services treating the many injured.
Damien Allemand, the paper’s correspondent, was quoted as saying: “People are running. It’s panic. He rode up on to the Prom and piled into the crowd . . . There are people covered in blood. There must be many injured.”
Social media carried images of people lying apparently lifeless in pools of blood and video of panicked people running through the streets to the sound of gunfire.
US government agencies have received regular reports of Isis threats to attack France and those threats are regarded as current, a US security official said. However, two US officials said they had no information at this point about whether militants were involved in the Nice incident.
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