Opening arguments began on Wednesday in the first criminal trial stemming from the January 6 assault on the US Capitol by Donald Trump supporters, a case that will be closely watched for its bearing on future prosecutions.

Guy Reffitt, 49, of Texas, is accused of bringing a gun to Washington, battling police officers, impeding an official proceeding and other offenses.

Prosecutor Jeffrey Nestler, in his opening statement, described the attack as the "worst assault on the Capitol since the War of 1812."

More than 750 people have been arrested around the United States for their roles in the failed bid by supporters of the former president to prevent Congress from certifying Democrat Joe Biden's victory in the November 2020 presidential election.

Nearly 220 people have pleaded guilty to various charges but Reffitt is the first person facing criminal charges to go on trial for their role in the violent storming of the Capitol.

Reffitt, who has pleaded not guilty, came to the demonstration with other members of a right-wing anti-government group called the Three Percenters, according to the affidavit supporting his arrest.

Photographs released by law enforcement showed Reffitt on the steps of the Capitol wearing a black motorcycle helmet and bulletproof vest.

In one picture, Reffitt is seen washing out his eyes with water after being repelled with pepper spray by police defending the Capitol.

According to the affidavit, Reffitt's teenaged son and daughter told the FBI that their father, on his return to Texas, had threatened them if they cooperated with law enforcement.

"If you turn me in, you're a traitor and you know what happens to traitors... traitors get shot," Reffitt was quoted as saying.

His children are expected to testify at the trial.

Reffitt was arrested on January 19, 2021 and FBI agents recovered an AR-15 rifle and Smith & Wesson pistol from his home in Wylie, a suburb of Dallas.

Reffitt, who has been jailed in Washington since his arrest, could face up to 20 years in prison if convicted.

His trial is expected to last at least a week. The jury is composed of nine men and seven women -- 12 jurors and four alternates.

The storming of the Capitol left at least five people dead and 140 police officers injured.

The maximum sentence meted out so far to a participant in the riot has been five years.