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The national president of the Oath Keepers, a prominent anti-government militia, is urging his members to stay away from a far-right demonstration in downtown Portland Saturday.

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The event is being billed as a rally against antifa. It's been condemned by Portland's mayor and a broad coalition of civil rights, law enforcement and business groups as a pretext for violence.

Related: Portland Community Groups, Mayor: White Supremacists Are Not Welcome Here

In a lengthy statement published online, Oath Keepers President Stewart Rhodes said people who attend the event risk associating themselves with white nationalists and could expose themselves to criminal charges for rioting.

“We do not, and cannot, knowingly associate with known or suspected white nationalists,” he wrote.

Rhodes said he does not consider the organizers of the event, a Florida man named Joe Biggs and Enrique Tarrio, the leader of the Proud Boys, a fraternity that describes itself as "western chauvinist" and espouses parts of white supremacist ideology, to be white nationalists themselves. But he faulted the men for not getting a permit for their march and failing to take other steps to exclude known white nationalists from joining it.

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“The left in this nation is desperate to paint all conservatives and, especially, all Trump supporters as 'white nationalists' to the point of absurdity. Allowing any actual white nationalists into an event where attendees are encouraged to bring Trump flags as well as American flags would be a grave mistake,” Rhodes wrote.

Rhodes also said he was concerned that angry videos and violent memes the organizers of the march have posted online suggest they want to incite violence and could expose attendees to prosecution for rioting under state and federal law.

As he urged organizers to cancel their event, an Indiana group that calls itself The American Guard confirmed it plans to attend.

Related: Portland Mayor Promises Heavy Response To Violence At Aug. 17 Protest

The group claims to promote states' rights and the U.S. Constitution. However, the group's president, Brien James, has a long history of founding neo-Nazi groups and has been involved in violent feuds between rival factions of racist skinheads, according to the Southern Poverty Law Center.

Twitter suspended Biggs's account in July. Antifascists have compiled a public archive of videos and memes posted by Biggs and members of the Vancouver, Washington-based Patriot Prayer, a far-right group that attracts white supremacists and has engaged in violence, including calls for murder and graphic depictions of violence against antifa.

In recent weeks, Biggs has publicly asked his supporters to tone down their rhetoric and refrain from showing up armed, after, he said, he was visited by the FBI.

Rhodes’ statement warning his followers to stay away from the demonstration comes as Portland’s mayor and police chief have promised a swift investigation of any crimes that take place.

In recent weeks, the Multnomah County district attorney has charged multiple members of the Patriot Prayer group with assault and harassment for their role in a brawl at the Portland pub Cider Riot in May.

Joey Gibson, Patriot Prayer's leader, has said he is turning himself in. He faces a felony charge of rioting. He has disputed the charge, calling it a politically motivated attempt to prevent him from participating in the demonstration Saturday.