Sewell, 29, leader of the European Australian Movement, faced Melbourne Magistrates Court on Tuesday after pleading not guilty to assault, affray and recklessly causing serious injury over the March 2021 incident.
Some of Sewell's supporters, observing the case remotely, were asked to turn on their video cameras. One said being asked to show his face was "abuse because you wouldn't ask a Muslim to remove their burqa" and another used a photo background of Hitler Youth.
Sewell is accused of punching a security guard in the head several times, outside Nine Network's headquarters in Melbourne on March 1.
He visited the office with a cameraman to speak to senior executives just hours before A Current Affair was due to air a segment featuring Sewell, about neo-Nazi groups.
He claimed ACA were showing his group to be a terrorist organisation and said they had not contacted him for right of reply.
A video of Sewell inside the building speaking to the camera was shown to court, along with CCTV footage of the fight.
"They call this investigative journalism but when anyone dares investigate these people they hide like cowards in their offices," he said in the clip.
The security guard then walks over and puts his hand over the camera lens, instructing the two men to leave.
He told the court he showed Sewell and the cameraman a white line outside the office where they were permitted to film, but they instead filmed close to the door. They eventually moved back and he believed the incident had de-escalated.
But then he alleged Sewell started playing the song Dance Monkey and said "the monkey is dancing" to the guard.
"Both men did refer to 'oh the monkey is dancing, the dance monkey'. I took that to be offensive," the security guard said on Tuesday.
"I said 'there's no need for any racial slurs' and then I hand gestured that I would appreciate if you would stop, I'm not a monkey, I don't appreciate the racial slurs, I never disrespected the both of you."
He alleged Sewell then king-hit him and he fell to the ground, hitting his heat on the concrete, and Sewell continued to punch him.
"I was held by my jacket collar, punched in the back of the neck, then continued to be punched repeatedly while I was trying to move away from Thomas Sewell," he said.
He came to after a few punches, was separated from Sewell and then walked back into the building. The guard suffered several injuries, including bruising and swelling to his head, and was left traumatised by the incident.
Sewell's lawyer James McQuillan claimed his client had acted in self-defence and was defending his cameraman when he attacked the man.
He alleged the security guard lost his temper and grabbed the cameraman by the throat after being told "dance monkey", which the guard denied.
Mr McQuillan also claimed no music was played and the guard was "imagining it or making it up", which the guard said was "totally wrong".
The hearing continues.
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