The leader of the far-right English Defence League (EDL) was arrested on Saturday after breaching a police order banning a march to the London site where a soldier was murdered last month, the group said.
Scotland Yard had warned Tommy Robinson and other EDL members they faced arrest if they went ahead with a planned walk and rally at the barracks in Woolwich, southeast London, where Lee Rigby was hacked to death.
Two British converts to Islam are due to stand trial over the murder in November.
Police said the EDL's plans risked causing "serious public disorder" and told the group to hold their rally, timed to mark Armed Forces Day in Britain, near parliament in central London.
The EDL campaigns against what it says is the spread of radical Islam in Britain. But it has been accused of Islamophobia and previous rallies have ended in clashes with anti-fascist groups.
Despite the police warning, Robinson went ahead with a sponsored walk through the capital with EDL co-leader Kevin Carroll, aimed to raise money for a children's cancer charity.
The movement said both were arrested as they entered Tower Hamlets, an east London borough with a sizeable Muslim community and a large mosque, on their way to Woolwich.
Footage on Sky News television showed Robinson being handcuffed and taken into a police van, although police could not immediately confirm the arrest.
"Tommy Robinson & Kev Caroll arrested for obstructing the police and carted off," the EDL said in a statement on its Facebook page, accusing police of acting like the Nazi Gestapo.
The group urged supporters "to continue the walk and lay the flowers at the scene of Lee Rigby's murder".
The government had already banned two US political bloggers from entering Britain to attend the EDL rally.
Pamela Geller and Robert Spencer, who set up Stop Islamization of America and operate the website Jihad Watch, were forbidden from entering Britain on the grounds their presence would "not be conducive to the public good".