The protest is taking place outside the US embassy in central London for the second time this week.
Thousands of protesters gathered outside the US embassy in central London on Saturday to protest a travel ban imposed by US President Donald Trump on citizens from seven mainly Muslim countries.
Demonstrators will also march on Downing Street, protesting against the British government's reluctance to condemn the ban.
Trump's executive order last week suspended the US refugee programme for 120 days and stopped visits by travellers from seven Muslim-majority countries: Iran, Iraq, Libya, Somalia, Sudan, Syria, and Yemen for 90 days.
A US federal judge on Friday put a nationwide block on Trump's executive order, although his administration could still have the policy put back into effect on appeal.
TRT World's Sarah Firth reports from London.
Protests across Australia
Thousands of protesters also rallied across Australia on Saturday.
About 1,000 people gathered in Sydney to protest against Trump's executive order on immigration and to call on Australia to close its offshore processing centres on the tiny Pacific Island of Nauru and Manus Island in Papua New Guinea.
Similar protests were held in Canberra, Newcastle and Hobart, while hundreds attended an anti-Trump rally in Melbourne on Friday.
In Sydney, protesters carried placards that said "Refugee torture, Australia's shame," and "No walls, no camps, no bans."