Police have arrested two men at the Unite the Kingdom march in London over an incident in Birmingham.
Officers detained the men in the vicinity of Euston station.
They were both wanted on suspicion of GBH following an incident in Birmingham where a man was run over.
The men were spotted arriving in London to attend the UTK protest.
It comes as the Crown Prosecution Service has issued new guidance on "offensive banners, slogans, chants or symbols" ahead of today's Unite the Kingdom rally.
In a message yesterday afternoon, the CPS made clear prosecutors would be cracking down on anyone "stirring up hatred" in the capital.
Two major rallies are set for later today: Tommy Robinson's Unite the Kingdom event, and a pro-Palestine Nakba Day march.
Prosecutors have been told to consider whether protest placards, banners and chants viewed on social media may amount to offences of stirring up hatred during the rallies.
Police have arrested two men at the Unite the Kingdom march in London over an incident in Birmingham
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It said the advice is designed to reflect "the changing international context".
The guidance also tells prosecutors to consider heightened tensions linked to national or international events.
"Some chants may amount to a criminal offence," the CPS said.
Already, Sir Keir Starmer has banned 11 foreign nationals from entering the UK who were due to speak at the Unite the Kingdom event.
Officers detained the men in the vicinity of Euston station
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Downing Street claimed the move was intended "to protect British communities from vile hate".
The rally, organised by Tommy Robinson and described as 'the greatest patriotic display the world has ever seen', will be attended by around 50,000 participants.
Some 30,000 more are set to march for Nakba Day.
That event marks the "Nakba" - a day of mourning for when Palestinians left or were expelled from the country after Israel declared independence in 1948.
The rally, organised by Tommy Robinson and described as 'the greatest patriotic display the world has ever seen', will be attended by around 50,000 participants
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Speaking during a visit to a Metropolitan Police command centre in Lambeth on Friday, the Prime Minister criticised the Unite the Kingdom organisers for "peddling hatred and division, plain and simple".
Sir Keir Starmer carried out his visit to Lambeth alongside Met Commissioner Sir Mark Rowley and London Mayor Sir Sadiq Khan before today's rallies.
Sir Mark, seated in front of live CCTV images of different parts of the capital, told the PM: “We’ve got a time when hate crime has been escalated for the last two or three years.”
He added: “And then ‘small p’ politics and protest groups who have got more polarised and angry, and so both groups at the weekend have a track record of having an intimidatory effect on the communities.”
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