French police secure the area as the investigation continues at the scene near the heavy truck that ran into a crowd at high speed killing scores who were celebrating the Bastille Day July 14 national holiday on the Promenade des Anglais in Nice, France, July 15, 2016.[Photo/Agencies]

Crowds of men, women and children were in the crowd when a 20-tonne white panel truck zigzagged through them at high speed on the fashionable Promenade des Anglais as they watched a firework display celebrating Bastille Day, which is France's national day.

Police shot dead the driver in an exchange of gunfire, eyewitnesses said.

Germany immediately tightened security along its borders with France under a previously agreed operation – both countries are part of the Schengen agreement, which permits freedom of movement within mainland Europe.

Belgium held a meeting of officials and said later there were fears of a second lone-wolf style attack using a vehicle on Belgian soil.

In the UK, newly appointed Prime Minister Theresa May said Britain stood "shoulder to shoulder" with France against what she called "these murderers," adding "I am shocked and saddened. These were innocent victims."

The BBC reported that UK intelligence officers were working with their French counterparts investigating the attack.

The UK is already on a high state of alert and intelligence chiefs attended a meeting of COBRA, the government committee that meets in times of crisis.

London mayor Sadiq Khan said security officials in the British capital were urgently reviewing security arrangements.

In France, President Francois Hollande, who flew to Nice to see things at first hand, said he had extended the state of emergency, first declared in November after terror bomb and gun attacks killed 180 and injured over 360 in and around Paris.

An extra 3,000 police and army reservists were being drafted in France to add to the 7,000 already on terror attack duty throughout France.

French Prime Minister Manuel Valls announced three days of national mourning, starting on Saturday, and in London officials flew the French tricolour flag at half-mast over the UK prime minister's official residence in Downing Street.

Valls said, in a statement: "Terrorism is a threat that is weighing heavily on France."

"We are faced with a war that terrorism has brought against us. The goal of the terrorists is to make us scared. We won't give in to the terrorist threat, we must stand together united. Times have changed, and we should learn to live with terrorism," he added,

Two days ago Patrick Calvar, head of France's DGSI intelligence service, told a parliamentary committee that at least one or two "lone wolf" style attacks, possibly using vehicles, were anticipated, with the danger rising ahead of the 2017 presidential elections in France.

To contact the reporter: chris@mail.chinadailyuk.com