Hollande discussed Trump with journalists in Paris, saying: "His excesses make you want to retch, even in the United States, especially when -- as was Donald Trump's case - he speaks ill of a soldier, of the memory of a soldier."
The French leader was referring to a feud between Trump and the Muslim parents of a slain US soldier, which has shaken the presidential campaign just three months before the November vote.
Hollande criticised Trump's "hurtful and humiliating comments".
"Democracy is also at stake, as we see more and more people tempted by authoritarianism," he said, "especially" in the United States.
"Should the American people choose Trump, there will be consequences, because a US election is a global election," he added.
