The Bastille Day attacks in Nice on Thursday has drawn international condemnation.
US President Barack Obama said it was a "horrific terrorist attack".
"On behalf of the American people, I condemn in the strongest terms what appears to be a horrific terrorist attack in Nice, France, which killed and wounded dozens of innocent civilians."
The United Nations Security Council said it "condemned in the strongest terms the barbaric and cowardly terrorist attack".
British Prime Minister Theresa May said she was "shocked and saddened" by the attack.
Mrs May added that she would be speaking to President Hollande today and would "make clear that the United Kingdom stands shoulder to shoulder with France today as we have done so often in the past."
Boris Johnson, Britain's new foreign secretary, said he was "shocked and saddened by the appalling events" in Nice.
Angela Merkel said she was shocked by "mass murder in Nice" as she promised to stand by France's side in the fight against terrorism.
"Germany stands in the fight against terrorism at France's side, united with many, many others.
"I am convinced that, despite all the difficulties, we shall win this fight."
Donald Tusk, the president of the European Council, paid respect to the victims at a summit for European and Asian leaders in Mongolia, which Merkel also attended.
"We stand united against violence and hatred," he tweeted.
Former France president Nicolas Sarkozy tweeted his "infinite sadness".
He added: "Deep emotion and infinite sadness at the attack #Nice06. Solidarity with #Nice and Alpes-Maritimes residents".
The two presumptive presidential candidates also reacted.
Donald Trump said "when will we learn?" in a tweet described as "opportunistic" by our Washington correspondent.
Hillary Clinton said the terrorists had struck one of America's closest allies and families "celebrating the history and culture of their country on Bastille Day".
"This cowardly attack only strengthens our commitment to our alliance and to defeating terrorism around the world. Our thoughts and prayers are with the families of those killed and injured, and with all our friends in France."
The Canadian Prime Minister Justin Trudeau said his people were shocked by the atrocity.