The leader of the far-right English Defence League (EDL) could face extradition to the US on criminal charges.
Tommy Robinson, whose real name is Stephen Yaxley-Lennon, was arrested amid reports that he entered the US illegally in September.
He appeared at Westminster magistrates' court charged with possession of an illegal identity document.
Yaxley-Lennon was remanded in custody until January to stand trial at Southwark crown court.
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He claimed on his Twitter page that his arrest stemmed from an invitation he accepted to speak in the US to mark the anniversary of 9/11 terror attacks.
On the movement's official website fundraising efforts for legal costs were under way.
"They want him removed from the United Kingdom of Great Britain to face a court in America," the website said.
"Tommy has been denied any help with legal aid. With Tommy having his accounts frozen he has no access to any monies for legal representation so we need your help."
In separate incidents, 53 people with suspected links to the EDL were arrested by police on the M25 and M3 motorways.
They were banned from entering east London by police as part of bail conditions.
The arrests came ahead of a planned march by the EDL in Walthamstow, east London, on 27 October.
Robinson used social media to publicise the event, revealing he intended to hold a viewing there of the inflammatory film Innocence of Muslims - which mocks Islam and portrays the prophet Mohammed as a paedophile.
EDL Leaders to Skip Provocative anti-Muslim March in Walthamstow, East London
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