Trump's attack on Khan came after the London mayor-who granted permission for protestors to fly the Trump baby blimp during Tuesday's mass demonstrations-accused the US president of behaving like "the fascists of the 20th century to garner support" and said he would join the U.K. Labour Party in boycotting the state visit.
After being greeted at Buckingham Palace with a cannon salute and marching Guards band, Trump had a private lunch with the monarch and tea with her son Prince Charles. She also showed him some of her family's golfing memorabilia in a nod to the president who owns two Scottish golf courses.
But most of the talk and the colorful images were just what the White House wanted to showcase Trump as a statesman while, back home, the race to succeed him - and talk of impeaching him - heated up.
TRT World's Simon McGregor-Wood brings more from London.
London Mayor Sadiq Khan speaks at the British Chamber of Commerce annual conference in London, Britain, March 28, 2019.
Earlier, Khan wrote in an editorial for The Guardian that Trump was one of the most "egregious examples" of the growing threat of rising nationalism and that it was "un-British to be rolling out the red carpet" for a U.S. president who was "seen as a figurehead of this global far-right movement".
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The Trumps' visit to the United Kingdom is attracting plenty of attention - and generating some controversy.
"He is a stone-cold loser who should focus on crime in London, not me".
".Kahn reminds me very much of our very dumb and incompetent Mayor of NYC, de Blasio, who has also done a bad job - only half his height". De Blaiso, a Democrat, is a longshot candidate in the 2020 presidential race.
Khan supporters have previously accused Trump of being racist against London's first Muslim mayor.
"He has been very positive about me and our country", Trump added.
So far there have been no gaffes by Trump like the one he committed previous year, when he breached royal protocol by turning his back on the queen and striding in front of her during a visit to Windsor Castle.
Donald Trump set to meet Tory leadership hopeful Michael Gove
But I would do so very much as a last resort, with a heavy heart, because of the risks to businesses and the risks to the union'. He says: 'A lot of teachers are telling me they feel the funding per pupil isn't high enough'.
In the Buckingham Palace ballroom with its six glittering chandeliers, the vast white-clothed horse shoe-shaped table has been laden with George IV's silver gilt Grand Service dinner set.
But the US president arrived at a precarious moment.
The state visit has been timed to coincide with the 75th anniversary of the D-Day landings, with both Trump and first lady Melania Trump expected to attend a special ceremony marking the occasion on Wednesday.
Protesters plan to fly a giant inflatable "Baby Trump" balloon, as well as a giant robot of Donald Trump sitting on a gold toilet shipped specifically for this event from China.
Trump's visit comes at a hard time for Britain, with Theresa May due to step down as prime minister within weeks, over her handling of Brexit. "I think that is the insult".
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It remains unclear exactly what the Trump administration would consider sufficient migration control to not impose the tariffs. Randy Mullett of Mullett Strategies told TT he believes the president is using the tariff threat as a negotiating tactic.