Simeon Boikov, who uses the nom de plume "Aussie Cossack", posted videos on his YouTube channel on Tuesday of him at the Town Hall rally.
Video appeared to show Boikov scuffling with an elderly man, who fell and was later taken to hospital with a head injury.
Police were called to the Town Hall after reports of a disturbance just after 1pm.
"Upon arrival, police were told a 76-year-old man was allegedly assaulted during a planned protest. The man was not seriously injured," police said in a statement.
Boikov was arrested at the scene and taken to Day Street Police Station.
He was charged with assault occasioning actual bodily harm and granted conditional bail to appear at Downing Centre Local Court on January 25.
In February, the 32-year-old stood as an independent candidate in the Strathfield by-election.
"As a nation, we need to come together, regardless of race, creed, ethnic or religious background," according to a statement posted on his website, Vote 1 for Simeon Boikov.
"We must band together to vote the major parties out of office. I support all efforts to remove Labor, Liberal, the Greens and the Nationals!"
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