More than 64,000 people signed up to demonstrate in London against the visit and other protests were expected in major cities around the country.
Supporters carried placards saying “Dump Trump” and “Keep your tiny hands off women’s rights” while others chanted “Donald Trump has got to go!”.
Earlier, activists floated a six-meter-high blimp outside parliament depicting the US president as an orange, snarling nappy-wearing baby.
“I wanted to make it very clear that, when Trump came to the country, that we don’t agree with his policies,” said Kimberley, 35, at the protests in central London. “I think it is important for him to see it … it will show the people that he is prejudiced against that we are behind them.”
Britain regards its close ties with the United States, which it calls the special relationship, as a keystone of foreign policy and Prime Minister Theresa May has courted Trump ahead of a projected departure from the European Union.
But some Britons see Trump as crude, volatile, unreliable and opposed to their values on a range of issues.
Trump, who arrived in Britain on Thursday, told the Sun newspaper that planned protests against him in London and other British cities made him feel unwelcome so he was avoiding the capital as much as possible.
He will steer clear of the demonstrations on Friday by meeting May and Queen Elizabeth outside London.
“I guess when they put out blimps to make me feel unwelcome, (there is) no reason for me to go to London,” he told the Sun newspaper. “I used to love London as a city. I haven’t been there in a long time. But when they make you feel unwelcome, why would I stay there?”