Enrique Tarrio, the former leader of the far-right Proud Boys militia, was sentenced to 22 years in prison for his role in the January 6, 2021 attack on the US Capitol, the stiffest sentence handed out so far.

"That day broke our previously unbroken tradition of peacefully transferring power," US District Judge Timothy Kelly said of the attempt to stop the congressional certification of Democrat Joe Biden's 2020 election victory over Donald Trump.

"Mr Tarrio was the ultimate leader of the conspiracy," Kelly said during a nearly four-hour sentencing hearing in the nation's capital.

Prosecutors had sought a 33-year prison term for Tarrio, who was not in Washington on January 6 but was accused of directing the military-style assault on the Capitol by members of the Proud Boys and other supporters of the former president.

The 39-year-old Tarrio and several other members of the Proud Boys were convicted of seditious conspiracy in May for their roles in the attack.

Another member of the Proud Boys, Ethan Nordean, 32, received an 18-year prison sentence from Kelly last week.

Stewart Rhodes, the founder of another far-right militia central to the Capitol siege, the Oath Keepers, was also sentenced to 18 years in prison earlier this year.

Tarrio addressed the court before the sentence was handed down and said January 6 was a "national embarrassment."

"My candidate lost," he said. "I persisted when I should have calmed."

Tarrio, who appeared to choke up on several occasions, apologized to members of law enforcement who were attacked by rioters on what he called a "horrible day."

"I failed miserably," he said. "This trial has humbled me."

Prosecutor Conor Mulroe, arguing for the 33-year prison term, said Tarrio and the other members of the Proud Boys "tried to use force and violence to try and impose their view of what was right for the country."

"It was Mr Tarrio with the assistance of his codefendants who put this group together," Mulroe said. 

Trump trial in March

In a sentencing memorandum, prosecutors described Tarrio as a "savvy propagandist" and said that while he was not physically present in Washington on January 6 he "did far more harm than he could have as an individual rioter."

Tarrio had acted as a "general rather than a soldier," they said.

"The only reason Tarrio did not march alongside the others is because he was arrested upon his arrival in Washington DC and placed under a court order to leave the District," they added.

The assault on Congress left at least five people dead and 140 police officers injured and followed a fiery speech by Trump to tens of thousands of his supporters near the White House in which he repeated his false claims that he won the election.

Trump is to go on trial in Washington in March on charges of conspiring to overturn the November 2020 election results.

He faces similar charges in a separate case in the southern state of Georgia.

The 77-year-old Trump was impeached for a second time by the House of Representatives after the Capitol riot - he was charged with inciting an insurrection - but was acquitted by the Senate.

More than 1,100 people have been charged by the Justice Department in the Capitol attack.

Some 630 of them have pleaded guilty to various charges, and 110 have been found guilty at trial.