IMAGE SOURCE, REUTERS Image caption, Robert Rundo was said to have been at a 2017 rally in Charlottesville that shocked the US
Robert Rundo, 33, co-founder of the Rise Above Movement, was arrested on Friday at a Bucharest gym following a tip-off, Romanian police said.
He was accused of violence in 2017 and indicted by a US court in 2018.
The case was dismissed in 2019, but a federal grand jury in Los Angeles issued a new indictment in January.
Romanian police told the BBC they detained Mr Rundo in response to the US legal move.
Mr Rundo and others were accused of rioting and conspiracy by recruiting and training members for violence and taking part in violence across the US in 2017.
Videos showed Mr Rundo punching counter-protesters in Huntington Beach and a police officer in Berkeley, according to an FBI affidavit.
In a statement, Romanian police said he allegedly conducted mixed martial arts training for members of the movement.
He also is accused of posting videos online of the group assaulting people at political meetings, the statement said.
Mr Rundo has also been active in Serbia and Bulgaria, and advocates the forming of small fighting groups with a violent neo-Nazi ideology.
