Tommy Robinson says he's been detained at Heathrow under counter-terrorism laws.
The far-right activist said his phones have been seized and he was held for "the best part of three hours".
"I'M A TERRORIST AGAIN," he posted on X.
"I was detained under section 3 of the Counter-Terrorism and Border Security Act 2019."
He added: "Looks like more defence and court fees ***!!!
"Absolute ****ing madness."
Robinson, whose real name is Stephen Yaxley-Lennon, earlier this month gave a speech at a protest in Southampton over the murder of 18-year-old Henry Nowak.
The teenage student was handcuffed by police who ignored his pleas that he had been stabbed as he lay dying after his killer Vickrum Digwa claimed to have been the victim of a racist attack.
During the Southampton protest, 13 police officers and a police dog were injured in clashes with demonstrators.
When he shared details of planned protests across the UK, Elon Musk then shared the post to his 240 million followers.
An admin on the former English Defence League leader's X account said: "This is an attack on free speech, this is an attack on investigative journalism, nothing more, nothing less."
The admin said officers "likely want to see who [Yaxley-Lennon] is talking to, and maybe find out who his sources are".
They claimed the activist's sources would "expose politicians".
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Robinson was previously cleared of a terror charge after he refused to give police the PIN to his phone during a stop in July 2024. He argued the device contained confidential journalistic material.
A district judge concluded he could not be sure the police stop, which happened at the Channel Tunnel while Robinson was driving to Benidorm, was lawful.
A Metropolitan Police spokesman said officers had stopped a man in his 40s at Heathrow Airport following his return to the UK from Russia via Turkey.
They said: "The man was interviewed by officers and his communication devices were seized. He was subsequently released."
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