"My message to those coming to the protests in London is that this must be peaceful and good-spirited".
Demonstrator Claudia Boes said she was pleased at the turnout, adding "even outside the normal political circles this is an issue that affects people".
At the same time, Trump will be touring a defense site with May outside of London.
Trump was joining British Prime Minister Theresa May for a black-tie dinner Thursday at Blenheim Palace, Churchill's birthplace near Oxford, at the start of his trip to England.
Protests are expected against Trump, whose policies - including a travel ban on a number of predominantly Muslim countries, the detention of child migrants on the US-Mexico border and the imposition of tariffs on European Union steel and aluminium exports - have all been criticised by the UK.
"I think it's amusing".
They are scheduled to spend Thursday night at the US Ambassador's home Winfield House, which is in Regent's Park.
Tens of thousands of people are expected to protest against Trump's visit in central London on Friday, citing his alleged Islamophobia, controversial policies and "incendiary rhetoric" towards people with disabilities and the LGBTQ community.
"He's come out of the woodwork now and I've seen it the last few days; there's huge positivity for what we've done here".
He is due to leave Britain on Sunday for talks in Helsinki the following day with Russian President Vladimir Putin, whose government stands accused by May's of unleashing a lethal nerve agent in the city of Salisbury.
Gardner, who works with refugees as part of her doctoral research at the London School of Economics, said Trump's speeches about refugees and immigrants are "a very risky line of rhetoric to be normalized".
But trip planners may go out of their way to shield Trump from viewing another aspect of the greeting: an oversized balloon depicting the president as an angry baby in a diaper that will be flown from London's Parliament Square during what are expected to be massive gatherings of protesters opposed to Trump's presence.
"I'd like to see them be able to work it out so it could go quickly".
In a statement ahead of Trump's arrival, she said the visit would focus on trade and strengthening defence and security ties, saying there was no stronger alliance than Britain's "special relationship with the U.S.".
On Friday evening, Trump and the First Lady whisk up to Scotland for a weekend of golf and relaxation - and more protests outside his resort. Many will see his avoidance of London as proof he is trying to keep away from dissent, however, the U.S. ambassador to the UK, Woody Johnson, denied this.
10 Downing Street said she would be meeting with schoolchildren and veterans on Friday, accompanied by Philip May, U.K. Prime Minister Theresa May's husband.
But there are still plenty of detractors, as loudly demonstrated by a passenger in vehicle driving past the USA flag-adorned pub who shouted "F*** Trump", to which Mr Smyth chuckled.