Echoing the words of Donald Trump, his lawyer D. John Sauer told the court that allowing the prosecution of a former president would open a "Pandora's box."

"To authorize the prosecution of a president for its official acts would open a Pandora's box from which this nation may never recover," Sauer said at the start of his prepared remarks.

"Could George W. Bush be prosecuted for obstruction of an official proceeding for allegedly giving false information to Congress to induce the nation to go to war in Iraq under false pretenses?" Sauer asked. "Can President Obama be potentially charged with murder for allegedly authorizing drone strikes targeting U.S. citizens located abroad?"

In response, Judge Florence Pan asked Sauer if presidential immunity could provide unlimited powers for actions outside a president's official acts.

"You're saying a president could sell pardons, could sell military secrets, could order SEAL Team Six to assassinate a political rival?" Pan said.

With all participants in place, the hearing has begun.

D. John Sauer, an attorney for Trump, is set to begin arguments on behalf of the former president.

Donald Trump's motorcade arrived at the courthouse at 9:15 a.m. ET.

He entered the building through an underground garage and did not make remarks.

Special counsel Jack Smith and his legal team have entered the courtroom for today's arguments before the D.C. Circuit Court of Appeals.

Smith is with his top prosecutor in the case, Thomas Windom, as well as Michael Dreeben, a former deputy solicitor general who recently joined Smith's team to assist in arguing through the appeals process.

Former President Trump has not yet entered the courtroom ahead of the 9:30 a.m. ET hearing.

Former President Trump again stressed that he is entitled to presidential immunity in remarks following his court appearance during today's hearing.

"A president has to have immunity," he told reporters. "And the other thing was, I did nothing wrong. We did nothing wrong."

"When they talk about the threat to democracy, that's your real threat to democracy," Trump said of the election interference case against him. "And I feel that as a president, you have to have immunity. Very simple. And if you don't, as an example of this case, where loss of immunity, and I did nothing wrong, absolutely nothing wrong -- I'm working for the country. And I worked very hard on voter fraud because we have to have free elections."

Trump warned that the case against him will set "a very bad precedent."

"It's a very bad thing. It's a very bad precedent," he said. "As we said, it's the opening up a Pandora's box and it's a very, that's a very sad thing that's happened with this whole situation."

-ABC News' Lalee Ibssa