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Tommy Robinson was arrested under anti-terrorism laws, according to his verified Twitter account.
It follows a complaint over a film played to the crowd at a march in central London on Saturday headed by Mr Robinson, real name Stephen Yaxley-Lennon.
According to a Twitter tweet, "We can confirm that Tommy Robinson was detained by police using powers granted to them under the Terrorism Act 2000."
"That's right, you read that correctly: Tommy is being held by police under counter-terrorism legislation.
"We will update you as and when we can."
The Metropolitan Police stated there was "no Met involvement" in the tweet concerning Robinson's detention.
Mr Robinson televised a one-hour, 45-minute "documentary" about a legal struggle.
In 2021 he was ordered to pay Jamal Hijazi £100,000 for allegations he made about the Syrian refugee after a video of him being attacked at his Huddersfield school went viral.
The false claims, including that Mr Hijazi had been attacking girls in his school, were repeated in the documentary.
