New Prime Minister Theresa May is being kept updated on reports of dozens dead after an apparent truck attack in the French city of Nice.
More than 70 people are reported to have been killed in the attack during a Bastille Day celebration.
A Downing Street spokesman said: "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 spokesman added: "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."
The Foreign Office issued a statement advising people in the area to "follow the instructions of the French authorities".
France is still on alert after the Paris terror attacks last November, in which 130 people were killed at sites including the Bataclan Theatre and Stade de France.
The Gendarmerie Nationale police authorities urged people to be calm and to look for trusted information rather than circulate rumours about the events in Nice.
The lorry ploughed through the crowd on Nice's Promenade des Anglais, which is famed as a landmark and runs for some four miles. Below it lies a rocky pebble beach.
It is a popular walking and cycling route and was busy with revellers celebrating and watching fireworks.