"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.
But on Friday Trump claimed that The Sun didn't include all his "tremendous" comments about May and explained his about-face by hinting that he had changed his mind on his criticism since speaking with May.
The president said the former foreign secretary would make a "great prime minister", adding "I think he's got what it takes".
Trump also used the interview to launch an attack on Theresa May's plans for Brexit which were unveiled in full yesterday and indicate a willingness on the UK's part to keep customs borders open with the EU.
She pressed Trump about the deal on Thursday at a gala dinner at Blenheim Palace, the birthplace of Britain's World War II leader Winston Churchill.
Trump even took a shot at London Mayor Sadiq Khan for not taking a harder line on immigration and painted the city as a hub for terrorism. "One of the reasons I got elected was because of immigration, I felt that Brexit had the upper hand, and most people didn't agree with me".
But well away from the noisy demonstrations, Mr Trump and Mrs May took questions from reporters.
As protests swarm across the country and a giant balloon depicting Mr Trump as a baby flies over London, this highly contentious visit is likely to determine US-UK relations going forward. Trump said the discussion "was really something".
Trump, who meets with Putin in Helsinki next week, said he would raise Russia's interference in the 2016 elections but added "I don't think you'll have any "Gee, I did it, I did it, you got me" " response from the Russian Federation leader.
"So I think allowing millions and millions of people to come into Europe is very, very sad", he said.
The President's remarks to The Sun materially damaged her biggest project on the eve of his visit, and he had gone out of his way to praise her political rival who had quit in protest at her Brexit plans earlier in the week.
Trump told the newspaper he had advised May on how to conduct Brexit negotiations, "but she didn't listen to me". "There will be no alliance more important in the years ahead", she said. "He thought she was great on North Atlantic Treaty Organisation today and is a really terrific person".
"I would have done it much differently", Trump said. We have more foreign investment in the United Kingdom going into the USA than all of Germany and France put together, a considerable multiple of the amount put in by the Chinese.
"But from the public, the welcome will be far from warm", he added saying there would be protests across the country against Trump's "abysmal record on human rights, his repugnant attitude towards women and his disgusting treatment of minorities".