Police estimate about 50,000 people will attend Tommy Robinson's Unite the Kingdom march, while 30,000 are expected to go to the pro-Palestine Nakba Day rally.
Armoured vehicles, police horses, dogs, drones and helicopters will be deployed along with around 4000 officers on duty as the Met aims to avoid clashes between the two rallies.
Crowds have gathered for the Unite the Kingdom march, many waving Union flags, while protesters in South Kensington are carrying Palestine flags and signs reading "smash the far right" for the pro-Palestine rally.
Chants of "Keir Starmer's a wanker" and "we want Starmer out" could be heard from the Unite the Kingdom march.
At the pro-Palestine march, a large group of protesters carrying banners and placards reading "Bristol stands with Palestine", "Stop Trump, Stop Farage", and "Free Palestinian Hostages" gathered with many wearing keffiyehs.
Robinson, whose real name is Stephen Yaxley-Lennon, posted on X on Saturday morning saying "today, we Unite The Kingdom and the West in the greatest patriotic display the world has ever seen".
Meanwhile, Daniel Kebede, National Education Union general secretary, said "we're marching today to show that we will not allow Tommy Robinson and the far right to divide our communities".
Sabby Dhalu, Stand Up To Racism co-convener, said: "Violence broke out the last time Tommy Robinson's Unite the Kingdom demonstration mobilised what was one of the largest far-right street demonstrations in recent British history.Â
"This comes amid wider attempts by figures such as Elon Musk to amplify far-right narratives and encourage political instability.
"The far right is attempting to exploit the cost-of-living crisis by stirring up racism and falsely blaming migrants and refugees for people's hardships in order to build a dangerous street movement.
"History shows where this kind of politics can lead."
Justice Secretary David Lammy said authorities would act "swiftly" if protests turned violent.
In a post on X on Saturday, he said: "The Unite the Kingdom march organisers are spreading hatred and division.
"They do not reflect the Britain I'm proud of. Peaceful protest is a fundamental right and one I will always protect.
"But if protest turns violent, we will act swiftly, with extra court capacity in place."
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