Sewell, 32, called out "we fight for white Australia" as he was led from the magistrates courtroom in Melbourne on Friday after the decision was handed down.
A dozen of his supporters, clad in black clothing, called back "heil Australia" from the public gallery.
Sewell is accused of leading a group of about 30 men to storm Camp Sovereignty in Melbourne's city centre following an anti-immigration protest.
It's alleged members of the white nationalist group, dressed in black, held down occupants of the Indigenous camp on Sunday evening, before kicking and punching them.
Three people were injured in the incident, with one woman requiring staples in her scalp to close a wound, the court was told.
Sewell was arrested outside court on Tuesday and charged with 25 offences including violent disorder and affray.
In applying for bail, Sewell's lawyer Mathew Hopkins argued the evidence against his client in the Camp Sovereignty incident was one-sided and weak.
But magistrate Donna Bakos on Friday found the case was a strong one, noting there was video of the alleged assaults.
"To lead and participate in an unprovoked attack ... can only be viewed as extremely serious conduct," she said.
Sewell was also charged over an assault on August 9 where he allegedly punched and tackled a man after he confronted Sewell as his white nationalist group marched in the city.
Ms Bakos accepted while there may be a defence for that incident, there could also be issues of using excessive force.
The magistrate also noted Sewell was on bail at the time of the alleged offending on charges of intimidation, which he is contesting in court.
He also has outstanding charges of behaving in an offensive manner and using offensive language over a series of marches he led in Ballarat and the Melbourne city centre.
Given his prior convictions for violent disorder and affray, Ms Bakos found Sewell could be facing a significant term of imprisonment if convicted on the fresh offending.
"The charges for which he seeks bail are another level," she told the court.
"I'm satisfied there is a risk he would, if released, endanger the safety and welfare of others."
In denying his bail application, Ms Bakos said Sewell was not charged with offences relating to his political views and ideology.
But she found he was a risk to the community and people had a right to go about their lives without being confronted by his hateful speech.
Sewell was remanded in custody to a committal mention date in December.
He is also due to return to Melbourne Magistrates Court on Monday for a contested hearing over the intimidation charges.
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