Miami police officers on horseback, motorcycles and bicycles are moving into positions around the downtown federal courthouse, while the Federal Protective Service is positioning cars by the building's entrance.

A few supporters of former President Donald Trump are outside the courthouse, some carrying flags and others in costume. Some opponents are in the crowd, as well, including a man carrying a "Lock Him Up" sign.

All demonstrators at this point are peaceful.

-ABC News' Brian Hartman, Aaron Katersky and Mara Robles

Former New Jersey Gov. Chris Christie took aim at former President Donald Trump during a CNN Republican Presidential Town Hall Monday night, calling Trump's conduct "inexcusable" for someone who wants to occupy the Oval Office.

"He has shown himself, and I think most particularly in his post-presidency, to be completely self-centered, completely self-consumed, and doesn't give a damn about the American people, in my view, if what the American people want is in conflict with what Donald Trump thinks is best for him," Christie said.

"I mean, put aside taking the documents in the first place," Christie said. "But then when you start getting asked … nicely with a letter from the archivist saying, 'Could you please give it back,' and you ignore it, ignore it, ignore it. Then they come with a grand jury subpoena, and then, according to the indictment, you tell your lawyers to tell them we don't have anything even though you have dozens and dozens of boxes of material. That's obstruction of justice, if it's true."

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Former President Donald Trump is expected to be joined by his lawyers Todd Blanche and Chris Kise at Tuesday's court appearance in MIami, sources told ABC News. Meetings with additional local attorneys are ongoing.

Blanche is representing Trump in the Manhattan criminal case while Kise represents Trump and the Trump Organization in other matters.

-ABC News' Katherine Faulders, Olivia Rubin and John Santucci

Former President Donald Trump is set to appear in federal court at 3 p.m. Tuesday.

He's accused of willfully retaining documents containing the nation's most sensitive secrets, including nuclear programs, after he left office, prosecutors said. He allegedly showed some of the documents to people on at least two occasions and then tried to obstruct the investigation into their whereabouts, prosecutors claim.

Trump denied any wrongdoing over the weekend, saying: "We did absolutely nothing wrong. Take a look at the Presidential Records Act. We did it by the book. Perfect."

Trump is expected to arrive at and depart from the Miami courthouse via secure private access points that would make it impossible for the public or journalists to see him.

Trump aide Walt Nauta, who was also charged in connection with his handling of government documents, is also due in court at 3 p.m.

Of all the federal charges that former President Donald Trump and his aide Walt Nauta face in the investigation into the alleged mishandling of top secret government documents, obstruction is one of the most serious, according to legal experts.

Claire Finkelstein, the founder and faculty director of the Center for Ethics and the Rule of Law at the University of Pennsylvania’s Annenberg Public Policy Center, noted that the obstruction charges in the indictment against Trump and his aide carry as much serious weight as the charges related to keeping the top secret documents, with a maximum penalty of 20 years in prison.

Finkelstein said an obstruction charge can cover a broad range of alleged activities, from as simple as lying to investigators, to as major as destroying evidence. But she said it all comes down to one clear allegation: that the accused deliberately interfered with an ongoing criminal investigation.

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-ABC News' Ivan Pereira