London's Metropolitan Police estimated that there were around 5,000 people at the protest.

With Trump receiving the red-carpet treatment at Windsor Castle, around 22 miles (35 kilometres) west of London, demonstrators marched in the heart of the British capital to display their disdain for him on the first full day of the trip.

"We're protesting, I suppose, about everything about Donald Trump. You don't know what placard to carry, really, there's so many things (to dislike)," former teacher Dave Lockett, 67, told AFP.

"He's sowing destruction and disorder throughout the whole Earth... If Trump's ideas get into this society, then what we're talking about is fascism in Britain," he added.

The protesters massed near the BBC's headquarters in the early afternoon, before heading south to Parliament Square for a rally hosted by well-known comedian Nish Kumar and podcaster Coco Khan featuring various speakers.

They were to include new Green Party leader Zack Polanski, veteran left-wing lawmaker Jeremy Corbyn and environmental activist writer George Monbiot.

Yashi Sriram, a doctor originally from India, turned out with a placard reading: "End the Genocide stop Trump".

"I just wanted to show support for the people of Palestine, really, more than anything else," said the 32-year-old.

"I don't think anyone should be welcoming Trump at this point. It's scary that he's the leader of a country to begin with," she added.

A group called the Stop Trump Coalition organised the demo, with a broad alliance of organisations sponsoring it, including Amnesty International UK, Black Lives Matter UK, Palestine Solidarity Campaign and Greenpeace.

- Heavy police presence -

The coalition questioned why Trump would barely be in London during the visit.

"Because he knows we're protesting against him," it said in a statement ahead of the march.

"Instead, he'll be hiding and having a sad little carriage ride all by himself in Windsor with a parade that no one will even see. This is because of the power of our protest."

London's Metropolitan Police launched a "significant" operation, deploying more than 1,600 officers -- including 500 brought in from other forces -- to ensure the demonstration went ahead safely.

Protests during Trump's first state visit in 2019 saw demonstrators floating a giant balloon depicting Trump wearing a nappy.

London mayor Sadiq Khan allowed the unflattering blimp -- which also featured in 2018 rallies against an earlier working visit by the president -- to fly, and it was seen as adding to a years-long feud between the pair.

Khan has been a persistent critic of the American president, writing Tuesday in The Guardian daily that "Trump and his coterie have perhaps done the most to fan the flames of divisive, far-right politics around the world in recent years".

It follows after an estimated 150,000 people attended a weekend rally in London organised by far-right activist Tommy Robinson, when 26 police officers were injured in clashes on the event's fringes.

In the article, Khan -- the first Muslim mayor of a Western capital when he was first elected in 2016 -- accused Trump of "scapegoating minorities, illegally deporting US citizens, deploying the military to the streets of diverse cities".

"These actions aren't just inconsistent with western values -- they're straight out of the autocrat's playbook," he wrote.

A lone counter-protester, surrounded by police, displayed a sign saying "We Love Trump", sparking boos as anti-Trump campaigners marched past him.