Committee Chair Bennie Thompson gaveled in the hearing shortly after 1 p.m. and immediately invoked a core theme the committee has emphasized in its public hearings.

"We settle our differences at the ballot box," he said, before raising how Trump handled his election loss.

"He seized on the anger of his supporters, and when they approached the line, he didn't wave them off, he urged them on," Thompson said.

"Today, the committee will explain how, as a part of the last-ditch effort to overturn the election and block the transfer of power, Donald Trump summoned a mob to Washington D.C. -- and ultimately, spurred that mob to wage a violent attack on our democracy," he said.

Capitol Police officer Harry Dunn, who testified at the first select committee hearing last fall on how he feared for his life and faced racist attacks while defending the Capitol on Jan. 6, told ABC News Congressional Correspondent Rachel Scott ahead of the hearing today that he's expecting the afternoon to be "triggering" -- and that he is "emotionally, preparing for the worst."

With Jan. 6 defendant Stephen Ayres set to testify, Dunn said Ayres "owes everyone in the congressional community who was affected by the day an apology." Adding, "if he stops short of being honest about the violence -- that doesn't do enough for me. If he stops short of apologizing -- that doesn't do enough for me."

Metropolitan Police Department officer Daniel Hodges, who also defended the Capitol and has also been a regular fixture at the public hearings, said it will be notable for Americans to hear what happened straight from someone who breached the building, given that some continue to downplay the violence.

"Having one of the people involved in the attack on Capitol -- in their own words describe their mentality, their intentions and the intentions of the group -- you can't get any closer to the source than that."

Video clips from the roughly eight-hour deposition committee investigators conducted with former Trump White House counsel Pat Cipollone last Friday are expected to be played at the afternoon hearing, a source familiar with the matter tells ABC News.

Committee member Rep. Zoe Lofgren, D-Calif., said last week after Cipollone was subpoenaed by the committee that his testimony did not contradict those of previous witnesses when he met with investigators.

Asked if Americans could assume that Cipollone confirmed the testimony offered by Cassidy Hutchinson, a former top aide to Trump's then-chief of staff Mark Meadows, Lofgren told CNN, "Not contradicting is not the same as confirming."

While Hutchinson publicly testified last month that Cipollone stressed to her that Trump should not be taken to the Capitol after his rally, warning, "we're going to get charged with every crime imaginable" if he went, according to Hutchinson, it was not clear if the committee asked Cipollone in his deposition about the comment.

-ABC News' John Santucci and Katherine Faulders

Today's hearing will partly focus on a meeting in the Oval Office on Dec. 18, 2020. Sources confirmed the meeting to ABC News at the time.

The meeting was said to be so long that it ended up moving from the Oval Office to the White House residence quarters upstairs. In attendance were Trump allies Sidney Powell; former CEO Patrick Byrne; former national security adviser Michael Flynn; then-White House counsel Pat Cipollone; then-chief of staff Mark Meadows and Rudy Giuliani, who joined by phone.

Powell, Flynn and Byrne argued with White House officials over invoking rarely used presidential powers to declare a national emergency to seize voting machines – a plan that was ultimately rejected. Trump in the meeting also discussed naming Powell a special counsel overseeing an investigation of voter fraud, as first reported by the New York Times at the time.

ABC News Chief Washington Correspondent Jonathan Karl described the meeting in his book "Betrayal" as one "so bizarre, long, and out of control that it may go down in history as the strangest meeting Donald Trump, or any other president, ever had at the White House."

- ABC News' Katherine Faulders and Will Steakin