Trump supporters clash with police and security forces as they storm the US Capitol in Washington, DC on January 6, 2021
Trump supporters clash with police and security forces as they storm the US Capitol in Washington, DC on January 6, 2021 AFP

A leader of the Proud Boys militia who called for a "war" to keep Donald Trump as president was sentenced to 17 years in prison Thursday, one of the longest sentences yet over the 2021 attack on the US Capitol.

Prosecutors said Joe Biggs was a key figure in a "seditious conspiracy" to forcibly overturn Joe Biden's election victory, leading Trump supporters in the January 6 military-style assault.

His sentence was one year short of the longest in the hundreds of convictions in the Capitol attack, an 18-year sentence given to Stewart Rhodes, the founder of another far-right militia central to the Capitol siege, the Oath Keepers.

But it was only half of the 33 years urged by prosecutors, as Judge Timothy Kelly said it was not a mass-casualty event and that Biggs had not intended to kill anyone in the uprising.

Nevertheless, Kelly said, "There is a need for deterrence."

The assault "broke our tradition of the peaceful transfer of power, which is one of the most precious things that we had as Americans," the judge said.

Before sentencing a tearful Biggs expressed regrets, saying he would have nothing more to do with the Proud Boys and would focus on his wife and daughter's welfare.

"I'm so sorry," he told the court.

"I know that I messed up that day, but I'm not a terrorist," he said.

Prosecutor Jason McCullough said what Biggs and his fellow rioters had done in shutting down the Congress that day was "no different than the act of a spectacular bombing of a building."

"They aimed to intimidate and terrify elected officials," he said, equating the January 6 attack to terrorism.

Biggs was the first of five Proud Boys figures, including national chairman Enrique Tarrio, to be sentenced this week.

Four were convicted on May 4 for seditious conspiracy while a fifth was found guilty of lesser charges.

The were all also convicted for obstruction of a congressional proceeding, impeding law enforcement and destruction of government property.

Biggs, leader of the Florida chapter of the Proud Boys, worked closely with Tarrio to organize the group to storm the seat of the US Congress.

Two months before, Biggs posted that it was time for "war," referring to Trump's loss to Joe Biden in the November 2020 presidential election.

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

Trump meanwhile faces separate conspiracy charges for his role in the buildup to the Capitol attack, which involved advancing a false claim that the election had been stolen from him.