The landmark sedition trial of five members of the far-right Oath Keepers opened on Monday with prosecutors telling a jury that the group heavily armed itself on January 6, 2021 to attack the Capitol to keep Donald Trump in the presidency.

Justice Department attorney Jeffrey Nestler said that Stewart Rhodes, the eyepatch-wearing former soldier and Yale law school graduate, knew exactly what he was doing when he led the militia's followers towards the Capitol.

Showing videos of the violent assault by dozens of group members dressed in military-style combat gear, Nestler said Rhodes directed them "like a general on the battlefield" as they sought to prevent 2020 election winner Joe Biden from being certified as the next president.

On January 6 the Oath Keepers "concocted a plan for an armed rebellion ... plotting to oppose by force the government of the United States," Nestler said.

"They did not go to the capital to defend or to help. They went to attack," he said.

But Rhodes' lawyer Phillip Linder, the first to present for the defence, rejected the government case, saying Rhodes had brought the Oath Keepers to Washington to provide security for Trump's speech that day and other pro-Trump events.

"The Oath Keepers are basically a peacekeeping force," said Linder.

"The real evidence is going to show our clients were there to do security for events that were scheduled for the 5th and 6th" of January 2021, he said.

They had created an armed "quick reaction force" on that day just in case they were needed – it would have been "defensive", he said, "if Trump called them in.

"Stewart Rhodes did not have any violent intent that day," he said.

The trial was the first in hundreds of cases from the January 6 riot to make use of the rare charge of seditious conspiracy.

Carrying a potential 20-year prison sentence, it is a gamble for the Justice Department, which wants to underscore the seriousness of the event, in which Trump supporters sought to prevent Congress from certifying Joe Biden as the victor in the November 2020 presidential election.

Out of 870 people charged so far, the government has reserved sedition for just a few dozen of the attackers, mostly members of self-styled militia groups like the Oath Keepers and Proud Boys who allegedly planned and coordinated the assault.

Four other Oath Keeper leaders are standing trial with Rhodes: Kelly Meggs, Thomas Caldwell, Jessica Watkins, and Kenneth Harrelson. Another four will undergo a separate trial.

Defence attorneys had argued in filings that the Oath Keepers believed Trump would invoke the 1807 "Insurrection Act," deputising them to protect the country.

Linder said Rhodes did believe the Insurrection Act could be invoked, but said he did not plan any attack on the Capitol.

"Rhodes is extremely patriotic.. he is a constitutional expert," the attorney said.

But Nestler said Rhodes told his followers that the possibility of Trump invoking the Act would provide legal protection for what they did at the Capitol.

"Rhodes' talk about the Insurrection Act was legal cover," Nestler said.

Despite being called on by many supporters to invoke it in the weeks and days before January 6, Trump in fact never did, Nestler noted.

Instead, Rhodes and the others spoke on encrypted chats of launching a civil war to prevent Joe Biden from becoming president.

"I'm personally gonna start the civil war myself," if Congress moves to certify Biden as president-elect, Caldwell wrote to the others, the jury was told. (AFP)