An elderly protester was hospitalised after sustaining a head injury during the incident.
Simeon Boikov, who posts online under the nom de plume Aussie Cossack, posted videos on his YouTube channel on Tuesday of him attending the rally at Sydney's Town Hall.
Video appeared to show Boikov in a scuffle with one male protester.
NSW Police told AAP officers 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.
"A 32-year-old man was arrested on scene and is now assisting police with inquiries."
Paramedics were called to the scene about 1.20pm to help the elderly man, who had suffered a head injury.
He was transported to Sydney Hospital for treatment.
Footage live-streamed from the rally shows Boikov arguing with protesters after the incident.
"Self defence. It's on camera," he told the group.
"That was self defence caught on camera. He approached me, he grabbed me, I pushed him away."
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