"His excesses make you want to retch, even in the United States, especially when — as in the case with Donald Trump — he speaks ill of a soldier, of the memory of a soldier," Hollande was quoted as saying by French media.
Hollande was referring to Trump's comments on the Khans, the family of a fallen American soldier. Those remarks have drawn criticism, both from Democrats and Republicans.
By calling those Trump remarks "hurtful and humiliating," Hollande became one of the candidate's most prominent foreign critics. Other European leaders, such as German Chancellor Angela Merkel, have refrained from publicly commenting on previous election campaigns. Asked about Trump at a news conference last week, Merkel only said she certainly had no nightmares about the possibility of him becoming U.S. president.
"I don’t want to wade into the American debate,” Merkel was quoted as saying, following her usual tactic to wait and see. "I’m following it with interest. I’ll just wait for the vote result."
But although Merkel's approval ratings are on the rise, her French counterpart is disliked by nearly 90 percent of his country, according to recent polls. A successful Trump could worsen his domestic problems, which might explain his harsh public criticism.
Hollande fears that the country's far-right party could gain power next year if Trump wins the election in November. General elections will take place in France in 2017, with the leader of the far-right National Front party, Marine Le Pen, as a main contender.
"If the Americans choose Trump, that will have consequences, because an American election is a world election," Hollande was quoted as saying, implying that a Trump presidency could have a contagion effect for right-wing movements across the globe.
"It could lead to a very strong turn to the right in the world," he said. "The American campaign shows issues that will be reflected in the French campaign."
Among those issues are France's increasing security fears, amid the threat of more terrorist attacks. Tensions with the country's immigrant community have been on the rise, and a weak economy has plagued the nation for years.
On both sides of the Atlantic, Le Pen and Trump have been accused of trying to capitalize on those issues.