From Trump impersonators to street scuffles and protestors chanting, "NO TRUMP, NO KKK, NO FASCIST USA", anti-Trump forces created quite a scene.
"We're also, in a light-hearted way, trying to articulate the strength of feeling against Donald Trump and his politics of hate", he said. His brother, Eric Trump, was part of the family's Tuesday tour of the underground Churchill War Rooms and lingered over the chair from which the prime minister oversaw World War II.
The leader of Britain's Labour Party, Jeremy Corbyn, fueled the crowds in the street with a passionate speech.
"Everything Trump stands for - misogyny, climate denying - everything about him is wrong", marble restorer Steve Gray told AFP.
The Trumps and the Mays met shortly before the UK PM will officially resign from her position.
He also held out the prospect of a meeting in the future with Jeremy Corbyn after snubbing him during this visit.
There, demonstrators are to fly the so-called "Trump baby balloon".
All the money in the world can't stop the truth, and even the world's most noxious liberal propagandists can't ruin President Trump's visit to the United Kingdom.
Also featured in Trafalgar Square: a 16-foot-tall, talking, animated "robot" of a plastic-cast Trump character, sitting on a golden toilet, his red tie dangling into bowl, tweeting. So it was fake news.
One supporter of the president wandered through the crowd wearing a "Make America Great Again" baseball cap, attracting puzzled glances.
Corbyn has faulted Trump's politics, and he was said to be addressing anti-Trump protesters in Central London around the time of the press conference between the USA and British leaders.
Matters are made worse by the fact that Brexit - the shock to the British establishment that closely paralleled the election of Donald Trump - just got another decisive "yes" from the British people.
"I think they are big powers over there, I think they have done a good job". "I think I'm making up for it rapidly because we're rebuilding our military at a level it's never seen before". "And that's OK for her to be nasty, it's not good for me to be nasty to her and I wasn't". But you saw the people waving the American flag, waving your flag. "There was great love", he said.
During a tour of a special collection of treasures at Buckingham Palace, Mr Trump failed when asked to recognise a polished pewter statue of a horse - a piece he had himself gifted to the Queen at tea with her at Windsor Castle previous year - but his wife Melania gamely came to the rescue.
Former prime minister David Cameron was left embarrassed in 2014 after he was overheard saying the monarch had "purred" when told the result of the Scottish independence referendum.
While the individual contributions to the protests are fun, there's one group that's doing the most. Trump called her a "washed up psycho".