Bob Geldof used his opening ceremony speech at the One Young World Summit to label Donald Trump a "racist" and a "liar".
Speaking in Ottawa, Canada, at a gathering of 1,300 current and future young leaders, Geldof grouped the Republican presidential candidate in a category of "commanders in the armies of stupid" that included Foreign Secretary Boris Johnson.
"Who believes any more that trade amongst the nations will lift us all out of the great evil that everyone has spoken of, poverty? And especially now that a liar, a fool and a racist vomits his bile on to the disinherited of the United States," the musician and activist said.
"And so, let us give up on belief. Let us surrender to the post-truth politicians, those reality television actors, the Trumps, the Putins, the Erdogans, the Brexiters.
"We must not succumb to the fools. We must not be galled by the awful thuggishness of Putin, Erdogan, Xi or Trump, or the many other commanders in the armies of stupidity.
"But the future is not binary, it's not that false choice of utopia or dystopia so beloved of journalists, commentators and writers.
"Rather it's a constant adjustment of circumstance ... that an impatient electorate or distracted populace is loath to accept or acknowledge, and thus they leave us to fools like Trump, or Johnson in the UK, or bullies like Putin."
Geldof, who was speaking at the seventh annual One Young World Summit, expressed a fear that the world's population no longer has faith in global institutions.
"I'm not sure any longer that people believe that action can be achieved through these great institutions.
"I believe that we are in some terrible moment, some great retreat from the ability to at least influence the direction of world travel," said the 64-year-old, on a stage that held former UN Secretary General Kofi Annan and the former Irish president Mary Robinson.
"Who believes now in the European Union when it is mired in repeated crises and seems incapable of reviving itself?
"Who now believes in the United Nations as the slaughter of Aleppo continues right now, as we meet in Ottawa?"
Geldof, who has spoken at every summit since the first in London in 2010, ended his speech with a question: "What is so funny about peace, love and understanding?"
One Young World features delegates from 196 countries and will tackle issues ranging from extremism to climate change over this week, with young leaders setting the agenda.