Theresa May says the UK must redouble its efforts to defeat terrorism, as she condemned "brutal murderers" behind "horrifying" attacks like that in Nice.
The prime minister said Britain stood "shoulder to shoulder" with France after a lorry drove into a crowd killing at least 84 and injuring 50.
Downing Street says a "small number" of Britons were injured in Nice.
The attack was of "an undeniable terrorist nature", French President Francois Hollande has said.
UK police forces are reviewing security at major events over the next week.
A meeting has been called of senior officials in the government's emergency Cobra committee to see what Britain can do to help, Mrs May said.
A number of Britons in Nice have described "horrendous" scenes as the lorry ploughed into packed crowds gathered for Bastille Day celebrations. The driver was later shot dead.
The Foreign Office advised any Britons in the area to contact friends and family to let them know they are safe.
The British embassy has opened a crisis centre with a 24 hour number: 0033 1 4451 3100. The French authorities have also set up a hotline: 0033 1 4317 5646.
Mrs May said she would speak to President Hollande "and make clear that the United Kingdom stands shoulder to shoulder with France today as we have done so often in the past".
"If, as we fear, this was a terrorist attack then we must redouble our efforts to defeat these brutal murderers who want to destroy our way of life," she said.
Mrs May said the government was "already making more funding available" for security services and policing, and senior officials would review procedures.
London Mayor Sadiq Khan said he would be reviewing the capital's safety measures in light of the attack.
The National Police Chiefs' Council is asking all UK forces to review major events over the next week to ensure "appropriate" security is in place.
Attacker identified
The attack in Nice's Promenade des Anglais happened at about 23:00 local time during celebrations to mark Bastille Day - France's national day.
French prosecutors said the lorry drove 2km (1.2 miles) through a large crowd, killing at least 84 and injuring about 50 people, 18 of them critically. Christian Estrosi, mayor of Nice, said about 10 children had been killed.
French media reports say the attacker has been identified as Mohamed Lahouaiej Bouhlel, who lived locally.
Paddy Mullan, from Northern Ireland, was standing on the promenade when the truck "came out nowhere" and started "ploughing" into the crowd.
He told BBC Radio Foyle: "This lorry just mounted the kerb, across the street from us and the next thing, all you could hear was banging and shouting and screaming."
'People running, screaming'
A former British serviceman, Will Shore, was in a nearby bar, and ran towards the scene to find out what was going on.
"It was quite chaotic really, there was a lot of people screaming, running around, 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."
Joel Fenster, a British tourist in Nice, told the BBC he hid behind a car alongside a mother trying to reassure her young daughter.
"And then later crouching in the restaurant with a whole load of young children as they're trying to keep them calm and of course they can't and they're crying - and watching that really was I think the most heartbreaking thing," he said.
Archbishop of Canterbury Justin Welby was among those to express his sorrow,tweeting: "As the French rejoice in their liberty, human evil kills the innocent cruelly. Let us weep with them, let us stand with them #PrayForNice."
People are leaving flowers at London's French embassy in Knightsbridge, where flags have also been lowered as a mark of respect to the victims. A French flag is also being flown at half mast above No 10.
