By Sarah N. Lynch

WASHINGTON (Reuters) - 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 January 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.

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 also 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 finalize a new trial date.

(Reporting by Sarah N. Lynch; Editing by Will Dunham)