PARIS — France’s national holiday turned bloody and violent Thursday night in the southern resort city of Nice, as a weapons-laden truck drove through crowds of Bastille Day revelers for more than a mile in an attack the French president labeled as terrorism.

The French Interior Ministry said at least 80 people were killed and at least 18 more critically wounded. By Friday morning, the death toll had climbed to 84. State Department spokesman John Kirby told The Associated Press that two Americans were among the dead, but he didn’t release the names yet, citing privacy reasons.

Family friend Jess Davis later told AP that 51-year-old Sean Copeland of Texas and his 11-year-old son Brodie were killed Thursday evening in the terror attack.

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The vehicle was a large white truck, similar to a tractor-trailer.

“France has been struck on the day of her national holiday … Bastille Day, the symbol of liberty,” President Francois Hollande said in a televised address to the nation after 3 a.m. Friday local time.

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Although there was no immediate claim of responsibility, the Paris prosecutor’s office quickly implied it was a terrorist attack by opening an investigation for “murder, attempted murder in an organized group linked to a terrorist enterprise.”

The president singled out Islamism and terror in his speech, which also announced several anti-terror countermeasures including extending a state of emergency. Mr. Hollande said he would visit Nice as an act of solidarity.

“France as a whole is under the threat of Islamic terrorism. We have to demonstrate absolute vigilance and show determination that is unfailing,” he said. “France is afflicted but I assure you she is strong and she will always be stronger than the fanatics that want to strike her today.”

Several French news outlets reported that ID papers in the truck showed the driver to be a 31-year-old Nice resident with dual French and Tunisian citizenship, therefore likely a French-born Muslim.

The Nice Matin, the city’s daily paper, identified the attacker as Mohamed Lahouaiej Bouhlel.

Eyewitnesses in Nice told CNN that it wasn’t merely a truck crashing into a building, bad as that can be, but a driver intentionally going onto pavement and barreling through at 20 or 30 mph for more than a mile on promenades packed with residents and tourists gathered to watch Bastille Day fireworks on France’s Mediterranean coast.

Sebastian Humbert, prefect for the Alpes-Martime area, said: “A truck rammed into the crowd over a long distance, which explains this extremely heavy toll.”

Wassim Bouhlel told The Associated Press in Nice that he saw a truck ram through the crowd before the driver exited wielding a gun and firing at random.

“There was carnage on the road,” Mr. Bouhlel told AP. “Bodies everywhere.”

The driver was killed in that ensuing gunfight, French officials said, and footage aired on several French TV news broadcasts showed the truck with its windshield riddled with bullet holes.

Some initial reports said a second assailant was in the vehicle and that man escaped armed. But those reports were not confirmed. By early Friday, it appeared the driver was alone.

Christian Estrosi, president of the governing council of the Provence-Alpes-Cote d’Azur region and a former Nice mayor, told the French TV network BFM that “the truck was loaded with arms, loaded with grenades.”

France’s national police force told Nice residents, “Emergency operation in progress. Keep calm and avoid downtown area. Follow the official accounts to be informed.”

“This is sad. Really sad,” said Chilani Kerdoni, 45, who was watching slack-jawed as the disturbing images moved across the television in the lobby of the Paris hotel where he works.

“I’m afraid. I’m shocked. We were just watching all the festivities in Paris and other places and then all of a sudden this happens in Nice,” he told The Washington Times. “Now, when something like this happens, we think automatically that it’s a terrorist attack. Even if we don’t know yet who did this, the first impression goes to terrorism.”

According to the Twitter account of the terrorism news portal TRACTerrorism.org, Islamic State-affiliated Telegram accounts were posting images from Nice. Islamist terrorist groups, sometimes using native French Muslims, have targeted France repeatedly in recent years, and the use of vehicles and car bombs is among their known tactics.

If Mr. Hollande is correct that the Nice incident was Islamist terrorism, it would fit with the predictions of several U.S. intelligence and law enforcement sources who have told The Washington Times in recent weeks that they are particularly wary that another wave of attacks by Islamic State operatives may be imminent in Western Europe.

The attack in Nice occurred as parades were being held in cities across France under tight security. A terrorist attack in France on July 14 would be as symbolic as one in the U.S. on July Fourth.

Bastille Day is France’s biggest public holiday, celebrating a Parisian mob’s storming of the eponymous royal prison on July 14, 1789, kicking off the French Revolution — the founding event of modern France.

France has been under an official state of emergency since November’s coordinated Paris attacks by the Islamic State, which left 130 people dead. France also has just finished Sunday hosting one of Europe’s biggest sporting events, the 2016 Euro soccer championships, which also prompted terrorism fears.

In his speech, Mr. Hollande said French authorities would extend the state of emergency, which had been scheduled to be lifted July 26, by three months.

He also said the government would mobilize military reservists to help with both internal policing and border control and would double down in its efforts against the Islamic State in the Middle East.

“We are going to strengthen our efforts in Syria and Iraq against those who are attacking us on our very soil,” he said.

Security was particularly tight at a grand military parade down the Champs Elysees in Paris, where members of the French intelligence services and Army marched for the annual Bastille Day celebrations.

U.S. Secretary of State John F. Kerry was among a host of special international guests at the parade.

In an interview on Fox News, presumptive Republican presidential nominee Donald Trump said the Nice attacks vindicated his tough stand on immigration and Islamism, even though the attack had been neither blamed nor claimed.

“We’re trying to be so politically correct and we’re making such big mistakes,” he said. “If it’s radical Islamic terrorism, [President Obama] ought to say it.”

He said that if he were president, “I would not allow people to come in from terrorist nations.”

In a statement released shortly after Mr. Trump’s interview, Mr. Obama called the attack “terrorism.”

“On behalf of the American people, I condemn in the strongest terms what appears to be a horrific terrorist attack in Nice, France. … We stand in solidarity and partnership with France, our oldest ally, as they respond to and recover from this attack,” he said.

Fernando Freitas, a Paris waiter who immigrated to France from Portugal in 2008, said he is somewhat worried this attack will cause backlash against immigrants in general, also without it being known who the perpetrator was.

“Maybe there is some chance it was not. But even if we don’t know, I’m thinking it’s the Islamic State,” said Mr. Freitas, 49.

Mr. Kerdoni, a Muslim who came to France 15 years ago from Morocco and has two children raised in France, told The Times that he already fears what politicians such as Marine Le Pen of the National Front will do.

“Yes. Every time this. … It’s a good opportunity for Marine Le Pen. … Ninety percent of the people are not against any Muslims here, but the right wing will profit from this even if the people who did this are not Muslim,” he said.

Mr. Kerdoni’s voice choked up as he said of the dead people: “They were trying to be happy on this night. They are dead now.”

• Victor Morton reported from Washington. Dave Boyer also contributed to this report from Washington.