London Mayor Sadiq Khan on Saturday denounced President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE as the “global poster-boy for white nationalism” in an essay that warns against forgetting the lessons of World War II.

“An entire generation of brave men and women around the globe sacrificed everything to defeat the singular evil of Nazism and fascism,” Khan wrote in The Guardian to mark 80 years since Nazi Germany invaded Poland.

His essay goes on to highlight the role Britain and other nations played in winning the war and establishing peace in the ensuing years. But he writes that he fears the lessons of World War II “are genuinely at risk of being forgotten or, worse still, being rewritten.”

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He notes that international bodies such as the European Union and NATO are facing "unprecedented attacks" and that "support for democracy is at a record low across the western world."

"This comes as a new wave of extremist far-right movements and political parties are winning power and influence at alarming speed – fuelled by Donald Trump, the global poster-boy for white nationalism," Khan argues. "Politicians across Europe are following his example by seeking to exploit division to gain power."

He later admonishes Trump for his attacks against the press and the judiciary, saying that those institutions are under daily verbal attack from the president "and other far-right leaders around the world."

"The impact can also be seen in the UK, where the outsize influence of Nigel Farage and his Brexit party has pushed the Conservatives, under Boris Johnson, to become ever more rightwing, illiberal and intolerant," he writes of the British prime minister.

Khan acknowledges that the current era isn't anything like the 1930s, but he stresses that "alarm bells should be sounding."

"We have a special responsibility to honour the memory of all those who sacrificed so much to protect us all those years ago – by defending the ideals they died for and ensuring the more peaceful and stable world they built lasts for generations to come," he concludes.

Khan has repeatedly criticized Trump, referring to him earlier this year as a "poster boy for the far-right movement around the world."

In June, he said that the U.K. was on the “wrong side of history" ahead of the president's visit to the country. The statement prompted Trump to call the Muslim mayor a "stone cold loser."