WASHINGTON — The House committee investigating the Jan. 6, 2021 attack on the Capitol plans to open a landmark series of public hearings on Thursday by playing previously unreleased video of former President Donald J. Trump’s top aides and family members testifying before its staff, as well as footage revealing the role of the Proud Boys, a right-wing extremist group, in the assault.

Committee aides say the evidence will show that Mr. Trump was at the center of a “coordinated, multi-step effort to overturn the results of the 2020 election” that resulted in a mob of his supporters storming the halls of Congress and disrupting the official electoral count that is a pivotal step in the peaceful transfer of U.S. presidential power.

The 8 p.m. prime-time hearing is the first in a series of six planned for this month, during which the panel will lay out for Americans the full magnitude and significance of Mr. Trump’s systematic drive to invalidate the 2020 election and remain in power.

“We’ll demonstrate the multipronged effort to overturn a presidential election, how one strategy to subvert the election led to another, culminating in a violent attack on our democracy,” said Representative Adam B. Schiff, Democrat of California and a member of the committee. “It’s an important story, and one that must be told to ensure it never happens again.”

The opening hearing will feature live testimony from a documentary filmmaker, Nick Quested, who was embedded with the Proud Boys during the storming of the building, and a Capitol Police officer, Caroline Edwards, who was injured as rioters breached barricades and stormed in.

The committee also plans to present what aides called a small but “meaningful” portion of the recorded interviews its investigators conducted with more than 1,000 witnesses, including senior Trump White House officials, campaign officials and Mr. Trump’s own family members.

Mr. Trump’s eldest daughter Ivanka Trump, his son-in-law Jared Kushner and his son Donald Trump Jr. are among the high-profile witnesses who have testified before the panel.

Mr. Quested, a British documentarian who has worked in war zones including Afghanistan, spent a good deal of the postelection period filming members of the Proud Boys, including the group’s former chairman, Enrique Tarrio, who has been charged with seditious conspiracy in connection with the riot. Mr. Quested accompanied the Proud Boys to pro-Trump rallies in Washington in both November and December 2020, and was on the ground with members of the group on Jan. 6, when several played a crucial role in breaching the Capitol.

Mr. Quested was also present with a camera crew on the day before the attack, when Mr. Tarrio met in an underground parking garage near the Capitol with a small group of pro-Trump activists, including Stewart Rhodes, the founder and leader of the Oath Keepers militia. Late in the day on Jan. 6, Mr. Quested and his crew were with Mr. Tarrio in Baltimore, filming him as he responded in real time to news about the riot.

Ms. Edwards, a well-respected Capitol Police officer, is believed to be the first officer injured in the attack on the Capitol, when she sustained a concussion during an assault at a barricade at the base of Capitol Hill. A man who has been charged with taking part in the assault, Ryan Samsel, told the F.B.I. during an interview more than a year ago that just before he approached the barricade, a high-ranking member of the Proud Boys, Joseph Biggs, had encouraged him to confront the police.

Other officers around the building recall hearing Officer Edwards, over the radio, calling for help — one of the first signs that mob violence was beginning to overrun the police presence. Months after the attack, she continued to have fainting spells believed to be connected to her injuries.

A committee aide said Mr. Quested and Officer Edwards would describe their experiences, including “what they saw and heard from the rioters who tried to occupy the Capitol and tried to stop the transfer of power.”

The committee’s investigators believe Mr. Quested overheard conversations among the Proud Boys during the planning for Jan. 6.

Representative Bennie Thompson, Democrat of Mississippi and the committee chairman, and Representative Liz Cheney, Republican of Wyoming and the vice chairwoman, are expected to lead the presentation of the committee’s evidence and question the witnesses.

The session will kick off an ambitious effort by the committee, formed last July after Republicans blocked the creation of a nonpartisan commission to investigate the attack, to lay out for Americans the full story of an unprecedented assault on U.S. democracy that led to a deadly riot, an impeachment and a crisis of confidence in the political system that continues to reverberate.

The hearings are unfolding five months before midterm elections in which the Democrats’ majority is at stake, at a time when they are eager to draw a sharp contrast between themselves and the Republicans who enabled and embraced Mr. Trump, including the members of Congress who abetted his efforts to overturn the election.

Other hearings are expected to focus on various aspects of the committee’s investigation, including Mr. Trump’s promotion of the lie that the election had been stolen, despite being told his claims were false; his attempts to misuse the Justice Department to help him cling to power; a pressure campaign on Vice President Mike Pence to throw out legitimate electoral votes for Joseph R. Biden Jr.; the way the mob was assembled, and how it descended on Washington D.C. on Jan. 6; and the fact that Mr. Trump did nothing to stop the violence for more than three hours while the assault was underway.

The committee has not yet committed to the full slate of witnesses for the six televised hearings, and is still discussing the possibility of public testimony with several prominent Trump-era officials.

Among the witnesses the committee has formally approached to testify next week are Jeffrey A. Rosen, the former acting attorney general, and Richard P. Donoghue, the former acting deputy attorney general, according to two people briefed on the matter.

Both Mr. Rosen and Mr. Donoghue have told multiple congressional committees that Mr. Trump and his allies pressured the department to falsely say that it had found voter fraud, and to use its power to undo the election results. Last May, Mr. Rosen took part in a public hearing of the House Oversight and Reform Committee on events leading up to the assault on the Capitol.

The committee is still in informal talks with Pat A. Cipollone, the former White House Counsel, as well as Byung J. Pak, the former U.S. attorney in Atlanta, who abruptly resigned on Jan. 4, 2021, after learning that Mr. Trump planned to fire him for not finding voter fraud, according to those people familiar with the discussions.

Mr. Cipollone would be able to speak on a range of issues, including Mr. Trump’s efforts to pressure the Justice Department and his unwillingness to accept the results of the election, despite the fact that officials time and again failed to uncover fraud.

Mr. Pak could have information pertaining to Georgia, a battleground state that Mr. Trump was particularly fixated on.

Alan Feuer and Katie Benner contributed reporting.