At least 84 people, including several children, have died after a terrorist drove a truck through crowds celebrating Bastille Day in Nice.

Eyewitnesses said the killer swerved from side to side to kill as many people as possible as he drove for hundreds of metres along the Promenade des Anglais on the seafront of the city on the French Riviera.

Hove conservative councillor Robert Nemeth was visiting the city with his girlfriend.

He tweeted:

New Foreign Secretary Boris Johnson said a Briton is believed to be among the scores injured.

Speaking to the BBC as he left his home, Mr Johnson said: "It's an absolutely appalling incident and there will be ministerial meetings later on today to discuss the implications.

"Clearly this represents a continuing threat. If this is a terrorist incident, as this appears to be, this represents a continuing threat to us in the whole of Europe and we must meet it together.

"The only information that I have is that there is one UK national who is injured."

Eyewitnesses in the resort said they assumed the driver had lost control as panic broke out.

But reports from the south of France suggested the driver - believed to be a 31-year-old French-Tunisian man - then produced a gun before being shot dead by police.

Witness Eric Ciotti told Europe 1 radio of the horrifying moments people fled to safety - with some running into the sea in a desperate attempt to flee.

He added: "A person jumped on to the truck to try to stop it. 

"It's at that moment that the police were able to neutralise this terrorist. I won't forget the look of this policewoman who intercepted the killer." 

Egyptian Nader El Shafei told the BBC he saw the driver "looking very nervous". 

He said: "I kept yelling at him, waving with my hands to stop and trying to tell him that there is a lot people under his truck - dead already. But he did not give any attention to anyone outside the truck. 

"And suddenly I saw him picking up something like a cellphone. I thought he would call the ambulance for the accident but it seemed that I was wrong, because he just picked up his gun and he started to shoot the police.

"Just when they (the police) arrived they just felt something was wrong so they kept yelling at him and when he did not step out - they saw him from the window taking his gun out.

"They knew that would be a gun shooting so they just killed him right away - they did not wait to negotiate or something, they just opened fire on him."

Christian Estrosi, the regional president in Nice, said at least 10 children were among the dead, and the death toll is expected to rise. 

French president Francois Hollande said the country's state of emergency would be extended for another three months. 

Mr Hollande led a wave of condemnation from world leaders, saying: "France has been hit by a tragedy once again. This monstrosity of using a lorry to deliberately kill people, many people, who only came out to celebrate their national day.

"France is in tears. It is hurting but it is strong, and she will be strong, always stronger than the fanatics who wish to hurt us."

He also said a military operation was in place which would allow the mobilisation of 10,000 troops, and that police from across the country would be called to assist their colleagues in Nice.

The country's borders were being tightened, said Mr Hollande, as he vowed that France would show "real force and military action in Syria and Iraq".

In a press conference at the scene, French interior minister Bernard Cazeneuve said: "We are in a war with terrorists who want to strike us at any price and in a very violent way."

Nice regional president Christian Estrosi said guns and grenades were found in the lorry.

The politician, who was at the celebration when the carnage happened, said: "This is the worst Nice drama of history.

"We are terrified and we want to present to all the families our sincere condolences."

France is still on alert after the Paris terror attacks on November 13, in which 130 people were killed at sites including the Bataclan Theatre and Stade de France. 

The state of emergency initiated after those attacks, which has now been extended, was due to be lifted later this month.

Lawyer Harjit Sarang and her children were among those caught up in the terror.

The Londoner tweeted: "Running through crowds in Nice with kids and terrified. Never taking kids to a public event again. Finally back to hotel. Hate this!"

She said it was the "scariest thing ever running through crowds with boys. Got back to hotel and couldn't get in for people seeking refuge!

"Can't stop shaking. Hate that my boys had to experience this. Why did I take them. Why did they do this and why the f*** is this happening!"

Briton Will Shore told BBC Radio 4: "There was a lot of people screaming, running around and people were kind of being pushed over, I think, from people just being so frightened about what was going on especially after hearing the gunshots.

"I had to help a couple of people up who were in distress on the floor because everyone was in such a panic."

Kevin Harris watched the attack unfold from his balcony.

He said: "It's a terrible scene."

Witness Lucy Nesbitt-Comaskey told Sky News: "I said to my friend 'This doesn't sound like fireworks, it sounds like Beirut when it's under fire'.

"All of a sudden people were screaming in the streets and running into all the restaurants.

"All the restaurants were open and people were coming.

"We were just sitting there and everyone came into our restaurant and the owners were saying 'Please don't go anywhere, come in, come in'."

Many families with children had gone to the seafront to watch the Bastille Day fireworks and in the aftermath adults were seen trying to comfort and calm terrified youngsters.

A Foreign Office spokeswoman said: "We are deeply concerned about reports of an incident in Nice and are in touch with the local authorities to seek more information.

"Our thoughts are with those affected and we stand by to help any British nationals."

A Downing Street spokesman said: "The Prime Minister is being kept updated on reports coming in from Nice.

"We are shocked and concerned by the scenes there. Our thoughts are with all those affected by this terrible incident on what was a day of national celebration.

"The FCO are in touch with the local authorities to seek more information and we stand ready to help any British nationals and to support our French partners."

Mr Johnson tweeted that he was "shocked and saddened by the appalling events in Nice, and the terrible loss of life".

A Rihanna concert scheduled for Friday night at the Allianz Riviera stadium in Nice has been cancelled, as have the city's jazz festival, which was due to run from Saturday to Wednesday, and some of the Bastille celebrations over the weekend.

The Association of British Travel Agents (Abta) issued a statement encouraging people to check with their tour operators before heading to France. 

A spokesman said: " Abta would recommend that all visitors read and follow the latest travel advice for France by talking to their travel agent or tour operator and following Foreign Office advice.

"As in the UK where the terrorism threat level is severe, travellers should remain alert and vigilant, especially in places where there is a high concentration of people, as advised by the UK Foreign & Commonwealth Office (FCO)."