Washington DC - US judge Hannah Dugan pleaded not guilty on Thursday to helping a man evade immigration agents, US media reported, as she argues that federal officials have no power to prosecute her.

Milwaukee County Judge Hannah Dugan was indicted Tuesday by a federal grand jury on charges of obstructing an official proceeding and concealing a person from arrest.

Her attorney issued a statement saying that Dugan "asserts her innocence and looks forward to being vindicated in court," according to the New York Times.

President Donald Trump has conducted a crackdown on undocumented migrants since coming to power in January, and has clashed with several courts for not following due process.

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Dugan's arrest set off a torrent of criticism by Democrats and applause by some Republicans, while more than 150 former state and federal judges signed a letter to the Justice Department calling the arrest an attempt to intimidate the judiciary.

Dugan is seeking to dismiss the case, arguing that she cannot be prosecuted over actions she took as a judge in and around her courtroom, according to local media.

Her lawyers have pointed to a Supreme Court decision last year that found that Trump was immune from prosecution for official acts.

The charges stem from a standoff with federal agents outside Dugan's courtroom in downtown Milwaukee in April.

They had come to arrest Eduardo Flores-Ruiz, a 30-year-old Mexican defendant appearing on misdemeanor battery charges, whom immigration officials say was deported in 2013.

Prosecutors say Dugan sent the agents down the hall to see the chief judge and, while they were away, postponed Flores-Ruiz's hearing and escorted him through a jury exit to a private part of the courthouse.

The area leads to the public hallway where immigration agents were waiting. But Flores-Ruiz was allowed to leave the building before being arrested outside.

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Dugan was arrested a week later in a sting publicized by FBI Director Kash Patel, who posted a photo on social media of her being led away in handcuffs.

Attorney General Pam Bondi accused Dugan of "protecting a criminal defendant over victims of crime."