Crowds gathered in Trafalgar Square on Saturday afternoon with banners which read “No to Maga hate” and “Dump Trump”.
The rally is one of hundreds of so-called “hands off” demonstrations around the world – including in cities across the US, Paris and Berlin.
The movement has been organised by more than 150 groups, including civil rights groups and trade unions.
In London, demonstrators voiced their concerns with chants of “Hands off Ukraine” and “Hands off the UK”.
Among the protesters was Clarke Reinstein, 73, from Rhode Island, who told the PA news agency he was “disappointed in my fellow Americans” over the election of Mr Trump.
“I’ve been in London for two months and I like being over here because people think a little differently,” he said
“They made a mistake over Brexit – but they didn’t fall over like the Americans have.”
A 37-year-old woman from Philadelphia was draped in the American flag that had been presented to her Second World War veteran grandfather.
The woman, who did not wish to be named, told PA she was protesting because of the “absolute, mind-blowing insipidness” of Mr Trump’s presidency.
“I don’t want to alienate the rest of the world,” she said.
“It’s not going to be useful for anyone in the long run. I felt powerless and I needed to come out and say something.
“I have friends in the United States who are marching in protest, and I wanted to march with them in solidarity.”
She explained that the flag she wore had been given to her grandfather, who served at Cambridgeshire airbase Duxford during the war.
“This is the flag they give a veteran when they pass away,” she said.
“My grandfather fought fascism and now to see it returning, he would be turning in his grave.”
Many of the protesters in the British capital are American expats.
Brent Robertson, 56, from Albuquerque, criticised Mr Trump’s tariffs.
A 25% levy on all foreign cars imported into America came into force on Thursday, and a wider “baseline” 10% tariff on goods imported from around the world kicked in on Saturday morning.
Mr Robertson said: “Stupid is not the word.
“He is shooting himself and all of us as Americans, just so he can make himself look like a tough guy.”
