Those angry at Trump are gathering outside the BBC’s offices in Portland Place, before marching down to Trafalgar Square for a rally at 5 p.m. local time (midday ET).
A further 10,000 people are expected to take part in a separate women’s march along exactly the same protest route, earlier on the same day. That demonstration will end with its own rally in Parliament Square between 2 p.m. and 4 p.m London time.
Popular opposition among Brits to an American president is not without precedent. The presidency of George W. Bush was marked with numerous protests in the U.K. in opposition to the Iraq War, and a mass protest in London in February of 2003 attracted more than 750,000 people, according to police estimates.
Despite today’s high-profile gatherings, many in the U.K. still remain supportive of Trump, particularly members of right-wing political parties. Trump’s higher profile friends in the country include former UKIP (U.K. Independence Party) leader Nigel Farage, Boris Johnson, and controversial businessman and political donor Arron Banks.
A recent YouGov poll revealed that while only 16 percent of Brits would have voted for Trump given the chance and 77 percent view him unfavorably, half of all respondents felt the visit should take place compared to 37 percent who wanted it cancelled, and 44 percent think the government should still try to work with him.
— CNBC’s David Reid contributed to this article.