Hutchinson described Tuesday conversations she had with Rudy Giuliani and then-White House chief of staff Mark Meadows on Jan. 2, 2021 -- four days before the attack on the U.S. Capitol.

Hutchinson said Giuliani said to her something "to the effect of 'We're going to the Capitol. It's going to be great. The president's going to be there. He's going to look powerful.'"

When Hutchinson went to Meadows' office to relay her discussion with Giuliani, Meadows told her: "There is a lot going on, Cass, I don't know, things might get real, real bad on Jan. 6."

"That evening was the first moment that I remember feeling scared and nervous for what could happen on Jan. 6," Hutchinson testified. "I had a deeper concern with what was happening with the planning aspects."

Introducing Hutchinson to the American people, Chairman Bennie Thompson asked Hutchinson to recall a typical day at the White House.

"When I moved over to the White House chief of staff's office with Mr. Meadows, when he became the fourth chief of staff, it's difficult to describe a typical day," she said.

Thompson established through a series of questions how Hutchinson's office was a five to 10-second walk from the Oval Office and that she regularly engaged with members of Congress and senior members of the Trump administration.

Vice chair Liz Cheney said Cassidy Hutchinson was in a "position to know a great deal about the happenings in the Trump White House."

"Today, you will hear Ms. Hutchinson relate certain first-hand observations about President Trump's conduct on Jan. 6," Cheney said in her opening statement. "You will also hear new information regarding the actions and statements of Mr. Trump's senior advisers that day, including his chief of staff Mark Meadows, and his White House counsel."

Cheney said information will also be released on what Trump and members of the White House knew about the potential for violence on Jan. 6.

Chair Bennie Thompson, D-Miss., convened the unexpected hearing shortly after 1 p.m. with Cassidy Hutchinson, a former top aide to Trump White House chief of staff Mark Meadows, taking her seat as members took the dais.

In brief opening remarks, he explained information that she had needed to be shared with the American people "immediately" and hailed her courage.

"In recent days, the select committee has obtained new information, dealing with what was going on in the White House on Jan. 6, and in the days prior. Specific, detailed information about what the former president and his top aides were doing and saying in those critical hours. Firsthand details of what transpired in the Office of the White House chief of staff," Thompson said.

"It hasn't always been easy to get that information, because the same people who drove the former president's pressure campaign to overturn the election are now trying to cover up the truth about January 6. But thanks to the courage of certain individuals, the truth won't be buried. The American people won't be left in the dark," he added. "Our witness today is Cassidy Hutchinson, she has embodied that courage."

It was among the shortest but most shocking Jan. 6 public hearings so far.

Cassidy Hutchinson, for nearly two-hours Tuesday, testified that Trump and Meadows were aware the Capitol was a target and that Trump supporters at the "Save America" rally were armed with weapons when the president told urged them to march to the Capitol on Jan. 6.

She said Trump told aides to let individuals with weapons past security and into the crowd, which he was "furious" about due to its size, with Hutchinson recalling Trump saying, "'I don't care that they have weapons. They are not here to hurt me. Take the effing mags (magnetometers) away.'"

Trump wanted to go to the Capitol himself after his speech, she said, and there was even conversation about having him go into the House chamber, despite the White House counsel's office raising serious legal concerns and the Secret Service raising safety concerns.

Still, demanding to go to the Capitol, Hutchinson recalled being told that Trump grabbed the steering wheel in the president's SUV -- on the way back to the White House and said, "'I'm the f---ing president. Take me up to the Capitol now!'" before lunging at a Secret Service agent.

Hutchinson also testified Trump instructed Meadows to make contact with a "war room" in the Willard Hotel on the evening of Jan. 5 and advised Meadows against going in person after hearing Rudy Giuliani's plans for the day, which she said she overheard included "Oath Keepers" and "Proud Boys."

In a statement to ABC News, Roger Stone said it was "FALSE" that he spoke to Meadows on the phone on Jan. 5 "or any other date."