At least 84 people are now believed to have died in a terror attack in the southern French city of Nice.
Children are among the dead after a lorry deliberately drove through crowds celebrating Bastille Day.
The lorry mounted the Promenade des Anglais and drove for up to two kilometres through pedestrians.
The driver was then shot dead by police.
Witnesses reported the lorry swerving across pavements in an apparent attempt to hit more people.
The attack happened at 23:30 last night as locals prepared for a firework display in honour of the country’s national day.
A state of emergency, originally implemented following a terror attack in November, has been extended for a further three months.
No group has so far said it was behind the attack.
However prosecutors said the inquiry would be handled by anti-terror investigators.
The British Foreign & Commonwealth Office warned travellers there continued to be a “high threat from terrorism” in France.
“Due to ongoing threats to France by Islamist terrorist groups, and recent French military intervention against Daesh (formerly referred to as ISIL), the French government has warned the public to be especially vigilant and has reinforced its security measures,” the FCO added.
Around 17 million British nationals visit France every year.
In a statement ABTA warned: “Following an attack on Bastille Day celebrations in Nice, travellers in Nice are advised to follow the instructions of French authorities, who are at present advising people to stay indoors at this time.
“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.”