Opening arguments began on Monday in the US sedition trial of five members of the far-right Oath Keepers militia, including its founder, who joined the 2021 attack on the Capitol in Washington.

Stewart Rhodes -- the eyepatch-wearing former soldier who plotted a military-style assault on the Capitol -- and his followers are charged with taking up arms against the United States to keep Donald Trump in the White House, despite his election defeat.

The Oath Keepers' attorneys are expected to argue that they believed Trump would invoke the 1807 "Insurrection Act," deputizing them to protect the country.

That claim has raised expectations that the trial could reveal more about links between the Capitol attack and members of Trump's administration or his personal advisors.

A jury was seated last week after Judge Amit Mehta rejected efforts by the defense to move the trial out of Washington on the grounds that residents are likely to be biased against the defendants because of the January 6, 2021 violence.

Rhodes's attorney has also asked the judge to forbid use of terms frequently used to describe the Oath Keepers -- such as "anti-government," "organized militia," "extremists," "racist" and "white nationalist" -- during the trial.

With a potential 20-year prison sentence, the sedition charge is the toughest yet in the prosecutions of hundreds who took part in the Capitol assault, which aimed to reverse President Joe Biden's victory in the November 2020 election.

Four other Oath Keepers are to go on trial beginning November 29.

Rhodes, a Yale Law School graduate, and his followers conspired "to oppose by force the lawful transfer of presidential power," according to the indictment.

At Rhodes's direction, "they equipped themselves with a variety of weapons," as well as combat and tactical gear, in preparation for January 6, it says.

"We aren't getting through this without civil war," Rhodes told the Oath Keepers in a group chat weeks before the assault on Congress, it adds.

The Oath Keepers are the first to go on trial for seditious conspiracy of the 870 people charged in the Capitol attack.

The majority have been charged with illegally entering the Capitol, illegally disrupting a session of the legislature -- the confirmation of Biden as president-elect -- and assault on law enforcement officers.

The sedition charge is rarely used by US prosecutors. The last time a conviction was obtained on the charge was against Ramzi Yousef, the planner of the 1993 World Trade Center bombing.

Members of the Proud Boys, another far-right group and key player on January 6, were also charged with seditious conspiracy in June, but their case has not gone to court yet.