The men allegedly belong to the "Boogaloo" movement, which has adopted Hawaiian shirts as a uniform and which promotes "a coming civil war and/or collapse of society," said a Nevada federal prosecutor.

The US has been roiled by nationwide protests against the killing of George Floyd by a white police officer in Minneapolis on May 25.

The demonstrations have been largely peaceful but many have broken down into violence and looting after nightfall. 

"Violent instigators have hijacked peaceful protests and demonstrations across the country, including Nevada, exploiting the real and legitimate outrage over Mr. Floyd's death for their own radical agendas," said the Nevada prosecutor, Nicholas Trutanich.

Reports of far-right activists, sometimes heavily armed, infiltrating the protests over the past week have included several claiming to be part of the "Boogaloo" movement.

Stephen Parshall, 35, Andrew Lynam, 23, and William Loomis, 40, all live in Las Vegas where they were arrested on Saturday by an anti-terror unit headed by the FBI.

They were in possession of a Molotov cocktail when they were detained, Trutanich said.

If convicted on federal charges the men face up to 30 years in prison. They were also indicted on terrorism conspiracy and other charges by state officials.