Recalling that visit during the press conference on Friday, he said there had been "an unbelievably large number of reporters there", adding: "I guess everybody was there because of Brexit".
"This is what people need to be doing - to come together in their communities to organize and work out how to stand against right-wing populism and xenophobia that we're seeing not just in the USA but in Europe", organizer Kevin Smith told The Associated Press.
In an antagonistic interview with The Sun, the USA president lashed out at Sadiq Khan, accusing him of doing a "terrible" job, claimed Boris Johnson would make a great Prime Minister and made it clear he was very aware of the giant inflatable Trump baby due to fly above Parliament today - and would be avoiding London.
Trump also heaped praise on Johnson, May's nemesis who resigned over the "soft Brexit" blueprint on Monday, suggesting he would "make a great Prime Minister".
More than 64,000 people signed up to demonstrate in London against the visit and other protests wre expected in major cities around the country.
Most of the vitriol in his interview was directed at London Mayor Sadiq Khan, who he said "has done a awful job" on terrorism and crime.
"I would have done it much differently", he told The Sun, which urged its readers to back Brexit before a referendum in June 2016.
Trump also denied a Washington Post report that he called May a "bossy schoolteacher".
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The White House did not say how much of the debt came from ticket purchases, or name the friends involved in the transactions. In 2016, Kavanaugh reported having between $60,000 and $200,000 in debt accrued over three credit cards and a loan.
Later on Friday, the president was due to pay a visit to Windsor Castle where he and First Lady Melania Trump will have tea with Queen Elizabeth II. "He obviously likes me, and says very good things about me", he said.
Conservative MP Sarah Wollaston said Mr Trump was "determined to insult" Mrs May.
One of the organisers behind "Baby Trump", Kevin Smith, said it was devised by a group of friends who got together in a pub. He also said a rival politician, Boris Johnson, would make a "great prime minister".
Last week at the same location, May finally won agreement for her Brexit strategy from her cabinet but within days, two senior ministers quit, which Trump said earlier in the week had left Britain in turmoil. "She didn't listen to me". Immigration was "very bad for Europe", he said, citing recent terror attacks. "Now he might not like the current president, but I represent the United States", Trump said.
"Take a look at the terrorism that is taking place".
"I think it changed the fabric of Europe and, unless you act very quickly, it's never going to be what it was and I don't mean that in a positive way", he said.
The man behind the balloon said the juvenility of his idea was a match for the juvenility of the president.