More than 4,000 police officers will be deployed across London as two protest marches and the FA Cup final take place on Saturday.
Tommy Robinson's Unite the Kingdom rally will coincide with the pro-Palestine Nakba Day rally, while Chelsea and Manchester City clash in the FA Cup final.
The policing operation is on an "unprecedented" scale, the Metropolitan Police said.
Heavy restrictions will be put in place to keep the two rallies separate, with police concerned about the prospect of some football fans going straight from the game to the Tommy Robinson rally.
For the first time under official protest restrictions, organisers and speakers both face prosecution if their rally is used as a platform for extremism or hate speech.
In another first for a policing operation around a protest, live facial recognition will be used, with cameras set up in a location in Camden, north London.
Deputy Assistant Commissioner James Harman said the operation will cost the force £4.5m, with £1.7m being spent to bring in officers from other forces to boost numbers.
He said Saturday "has the potential to be one of the busiest days for policing in London in recent years".
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Calling the scale of the operation is "unprecedented in recent years", he described both protests as "significant and potentially challenging".
The terrorism threat level had been raised to severe, he added, while noting an increase in hate crimes - particularly targeting Jewish Londoners.
"Taken together, these factors give us significant cause for concern as we head into the weekend," he said.
It's a situation, he said, with "potential for serious disorder and other criminality".
Some 50,000 people will attend the Unite the Kingdom rally, with 30,000 more at Nakba Day, police estimate.
(c) Sky News 2026: London faces 'unprecedented' police operation when rival rallies held on day of FA Cup final
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