The leader and founder of the right-wing Oath Keepers militia Stewart Rhodes received an 18-year prison sentence for seditious conspiracy in the 2021 attack on the US Capitol, the toughest penalty given yet over the January 6 assault.

"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 supporters of then-president Donald Trump.

"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".

Although Mehta agreed with the notion that the Oath Keepers' attempt to physically prevent Joe Biden from becoming president amounted to terrorism, the sentence fell short of the 25-year penalty requested by the government.

Just before his imprisonment, Rhodes vehemently defended his gang and their acts in support of Trump while wearing an eye patch and clothed in his orange jail uniform.

"I'm a political prisoner," he declared, comparing himself to the famed Soviet dissident Aleksandr Solzhenitsyn. 

"My only crime is opposing those destroying our country," he said.

The investigation into the insurrection that took place on January 6 has so far seen more than 725 individuals from nearly all 50 US states charged with crimes related to breaching the Capitol.

Read next: Leader of Oath Keepers group arrested in January 6 probe