Thousands of Tommy Robinson supporters have gathered to protest in central London.
The far-right activist, whose real name is Stephen Yaxley Lennon, rallied his supporters on social media early on Saturday, claiming: “Today the people of Britain will send a message to the world.
“We will remain silent no longer. WE WANT OUR COUNTRY BACK.”
He described the event as the “biggest patriotic rally the UK has ever seen”.
A counter-demonstration has been organised by former Labour Party leader Jeremy Corbyn’s Peace & Justice project and the organisation Stand Up To Racism.
The counter-protesters are expected to march from Russell Square to Whitehall.
They are being supported by a number of trade unions, the Stop The War coalition, and Unite Against Fascism, among other groups.
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Mr Robinson’s supporters filled up The Strand waving the flags of England, Scotland, Wales, and the UK outside the Royal Courts of Justice on Saturday.
Chants were heard of “Rule Britannia”, Mr Robinson’s name, and “we want our country back”.
One man was seen with a T-shirt that read: “I identify as English”, while others had Donald Trump merchandise.
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The rally is expected to move to Trafalgar Square, with around 1,000 police officers deployed to “keep the peace”.
Metropolitan Police Chief Superintendent Colin Wingrove, who is in charge of this weekend’s operation, warned that police will “intervene decisively” to deal with protesters who break the strict conditions about when and where they are allowed to demonstrate.
He added a “significant number” of officers are on duty as part of “a carefully planned operation” in an effort to keep people safe and prevent crime.