Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

A truck plowed into pedestrians during Bastille Day celebrations in the popular French seaside city of Nice Thursday, leaving at least 80 people dead in what the nation’s president called "obviously a terrorist attack."

The deadly toll, which included several children, came after the truck slammed into revelers gathered on a promenade to watch fireworks, French President Francois Hollande said in an address Friday morning.

"Such a monstrosity," Hollande said. He pledged to step up efforts to fight terror in Iraq and Syria, and said he will seek to extend a state of emergency by three months.

"France is deeply saddened, but it is also very strong," Hollande said. "I can assure you we will always be stronger than the fanatics who are trying to attack us."

The driver was fatally shot by police, Hollande said. It is unknown if there were accomplices, Hollande said. French media, citing a police source, reported that ID papers belonging to a French-Tunisian were found in the truck. A source told NBC News the driver is believed to have been a French national of Tunisian descent

The truck struck the crowd at around 10:40 p.m. local time (4:40 p.m. ET) shortly after a fireworks display, officials and witnesses said.

Christian Estrosi, president of the Provence-Alpes-Côte-d'Azur region that includes Nice, told French media that the driver also shot at people. A senior U.S. military official told NBC News that guns and explosives were found in the truck.

Related: Live Updates on Truck Attack in Nice, France

Eyewitness Andrew Botros described the scene as a "white truck literally racing through crowds of all ages." The ranking politician of the Alpes-Maritimes department that includes Nice said the truck plowed into the crowd over a distance of 1.2 miles.

At the time of Hollande's address, 77 people were confirmed dead. A short time later, the French interior minister said the death toll had risen to 80, and 18 others were in critical condition.

U.S. President Barack Obama earlier condemned what he called "what appears to be a horrific terrorist attack" and said the U.S. has offered any assistance France may need. "We stand in solidarity and partnership with France, our oldest ally, as they respond to and recover from this attack."

Witnesses described scenes of chaos after the truck hit the crowd.

"I looked up and saw, like, a tsunami wave of people just running towards me as fast as they possibly could screaming at the top of their lungs," Dr. Kevin Motamedi, a Denver physician on a tour of Europe, told NBC News. “And I just grabbed who I was with and started running as fast as possible. It was basically just complete mass hysteria.”

"It was the scariest moment of my life, easily," Motamedi said. "As we were running, you could tell the people in front of us had no idea that anything was going on, so we were running and grabbing people saying, 'Run. Go home!'"

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Related: Witnesses Relate Horror of Deadly French Crash

Eric Dratell, an American lawyer working in London, is in Nice on vacation with his wife. They were having dinner at Le Sporting on the beach when his wife heard what she thought was gunfire.

"We started running for cover. People started jumping from promenade level onto the beach," Dratell told NBC News. "A guy jumped on my wife," who was injured, he said.

"We took shelter with 200 or more people in an area under the promenade. People were in crowded toilet stalls," Dratell said. "This is shocking," he said.

In the immediate aftermath of the crash, Estrosi, who is also Nice's former mayor, tweeted in French: "Dear Nicois, the driver of a truck appears to have made a dozens of deaths. For the moment, stay in your home. More info to come."

Related: A Short History of Vehicles Being Used as Weapons

France has been on edge since a series of coordinated terror attacks in Paris in November that left 130 people dead. The terror group ISIS claimed responsibility for that attack.

There were no immediate claims of responsibility by ISIS through its official or unofficial channels, according to Flashpoint Intelligence, an international terrorism research organization.

Hollande said he will be calling up "operational reserves" to help police and a Sentinel operation allowing 10,000 troops to be on patrol would be maintained. He said the country’s military campaign against the terror group ISIS in Iraq and Syria would increase.

"Nothing will shake us and make us renounce the fight against terrorism,” Hollande said.

Police officers and rescued workers stand near a van that struck a crowd leaving a fireworks display in the French Riviera town of Nice on July 14, 2016. VALERY HACHE / AFP - Getty Images

France’s ambassador to the U.S., Gérard Araud, said on Twitter: "Again. Sadness. These people only wanted to enjoy Bastille day fireworks with their family and friends. Sadness."

Canadian Prime Minister Justin Trudeau said: "Canadians are shocked by tonight’s attack in Nice. Our sympathy is with the victims, and our solidarity with the French people."

Related: World Reacts to Deadly Attack in France

The United Nations Security Council "condemned in the strongest terms the barbaric and cowardly terrorist attack which took place in Nice," its members said in a statement.

Nice is a city on France’s Mediterranean coast popular with tourists. The U.S. State Department said it has no Thursday evening it had no information that any Americans were killed or injured.

U.S. Secretary of State John Kerry encouraged any Americans in Nice to contact friends and family. Facebook activated its Safety Check feature, which allows people to alert loved ones their whereabouts.

Bastille Day, also known as National Day, commemorates the storming of the Bastille prison during the French revolution in 1789.

"On this Bastille Day, we are reminded of the extraordinary resilience and democratic values that have made France an inspiration to the entire world, and we know that the character of the French Republic will endure long after this devastating and tragic loss of life," Obama said.