The president called the mayor a “stone cold loser”, adding: “Kahn reminds me very much of our very dumb and incompetent Mayor of NYC, (Bill) de Blasio, who has also done a terrible job – only half his height.
“In any event, I look forward to being a great friend to the United Kingdom,” he added.
Queen Elizabeth II was scheduled to welcome Trump and his wife Melania to Buckingham Palace later Monday, where they will be treated to a guard of honour, a private lunch and a glittering banquet.
But beneath the pomp and ceremony, Britain is in turmoil with Prime Minister Theresa May due to step down within weeks over her handling of her country’s exit from the European Union.
Trump weighed in on the divisive issue of Brexit, declaring before he arrived that Britain’s former foreign minister Boris Johnson would make an “excellent” choice to succeed May.
In a round of British newspaper interviews, he also recommended her successor walk away from talks with Brussels, refuse to pay Britain’s agreed divorce bill and leave the EU with no deal.
“Donald Trump is just one of the most egregious examples of a growing global threat,” Khan wrote.
His spokesman called Trump’s tweets “childish” and “beneath the president of the United States”.