Washington (AFP) - The founder of the far-right Oath Keepers militia was handed an 18-year prison sentence Thursday for seditious conspiracy in the 2021 attack on the US Capitol, the toughest penalty given yet over the January 6, 2021 insurrection.

Elmer Stewart Rhodes was one of more than 1,000 people charged over the attack which, encouraged by then-president Donald Trump, aimed to block Congress from certifying Joe Biden as the winner of the November 2020 election.

"Seditious conspiracy is among the most serious crimes an American can commit," said Judge Amit Mehta in pronouncing the sentence.

"You present an ongoing threat and a peril to this country," Mehta told Rhodes, who led the Oath Keepers and organized their participation, with a stockpile of arms, in the attack on the Capitol by Trump supporters.

"You are smart, charismatic and compelling and that is frankly what makes you dangerous," Mehta said -- rejecting Rhodes' claim that he was a "political prisoner."

The sentence fell short of the 25 years the government had sought, although Mehta accepted the argument that the Oath Keepers' plan to violently block Biden from becoming president amounted to terrorism.

Just ahead of the sentence, Rhodes, wearing an eye patch and dressed in his orange prison jumpsuit, defiantly defended his group and their actions in support of Trump.

"My only crime is opposing those destroying our country," he declared, comparing himself to the famed Soviet dissident Aleksandr Solzhenitsyn.

Blame Trump

But his group's stockpiling weapons just outside the city and wearing combat-style gear in an organized push into the building showed a level of planning and preparation for violence not present with many of the others in the crowd.

Rhodes, 57, and Kelly Meggs, 53, leader of the Oath Keepers' Florida chapter, were convicted by a Washington jury in November of the rarely pursued charge of seditious conspiracy -- plotting to overthrow the government or unlawfully opposing its authority.

In the same trial, three other Oath Keepers were convicted of obstructing an official proceeding, as the rioters shut down the Congress and sent lawmakers and vice president Mike Pence fleeing to safety.

During the trial, prosecutors said the Oath Keepers "concocted a plan for an armed rebellion... plotting to oppose by force the government of the United States."

Rhodes' attorneys argued that he himself never entered the Capitol building and that he did not support others doing so.

But Mehta rejected that as mitigating the sentence.

Rhodes was unequivocally the leader of the group and summoned them to Washington with a cache of arms for the violent assault, Mehta said.

"Stewart Rodes is a Yale Law grad and a pretty smart guy," the judge said. "He was the one giving the orders... They were there because of him."

Rhodes' attorney Phillip Linder however said he should not be held responsible for the Capitol attack and pointed his finger at Trump.

"I think what happened on January 6 was deplorable," Linder told the court.

But Rhodes did not plan the uprising, he insisted.

"We need to look at what caused this... Who got the Million Maga rally started?... Who got January 6 started?" Linder said.

"He's not the one that started that rhetoric that got the people ginned up."