Enrique Tarrio, the former top leader of the right-wing group the Proud Boys, pleaded not guilty on Tuesday to criminal charges accusing him of conspiring to block Congress from certifying President Joe Biden's 2020 election victory ahead of last year's attack on the U.S. Capitol.

Tarrio's attorney entered the not-guilty plea on his behalf during a virtual hearing in the U.S. District Court for the District of Columbia. The judge also announced a postponement in the planned May 18 start to the trial.

Tarrio, 38, is one of the most high-profile of the more than 775 people criminally charged for their roles in the Jan. 6, 2021, attack on the Capitol by supporters of then-president Donald Trump on the day Congress convened to certify Biden's victory. Tarrio and five other members of the Proud Boys are charged with conspiring to block the certification.

Tarrio himself was not present at the time of the attack, unlike his co-defendants. Prosecutors said Tarrio still maintained an active leadership role behind the scenes.

He was arrested on March 8, and remains in jail, after a federal magistrate judge in Florida ruled that he presents a danger to the community if released.

Members of various right-wing organizations have been arrested in connection with the Capitol attack, also including the Oath Keepers militia and its leader Stewart Rhodes.

During Tuesday's hearing, U.S. District Judge Timothy Kelly also said he will be granting a request by the government to postpone the trial date and added that he will release a written ruling later in the day.

Tarrio was added last month as a co-defendant in the case, and prosecutors indicated there is still a chance they could file additional charges or charge more defendants.

With a packed trial schedule ahead for the court, both because of the number of cases and due to delays from the COVID-19 pandemic, Kelly said he will hold off on selecting a new date until attorneys in the case can confer with one another.

He said he will schedule another court hearing for April 21 to hopefully finalize a new trial date.

In addition to the criminal cases, a Democratic Party-led select committee in the House is investigating "the facts, circumstances, and causes," related to the Jan. 6, 2021, attack.

On Tuesday, Ivanka Trump was set to appear before the committee, according to three sources who spoke to the Associated Press on the condition of anonymity. The former president's daughter was among those closest to him during the insurrection at the Capitol, and her committee appearance would follow that of her husband, Jared Kushner, last week.

The committee on Monday moved to advance contempt proceedings with Trump administration figures Peter Navarro and Dan Scavino, something the panel had earlier undertaken with unco-operative former chief of staff Mark Meadows. Trump ally Steve Bannon's refusal to co-operate with the committee has led to a federal indictment.

The committee has interviewed over 800 people and plans a round of public hearings in the spring. Republicans aside from Liz Cheney and Adam Kinzinger have panned the committee, and Democrats are working hard to complete their work ahead of November midterm elections that could see them lose control of the chamber.