Eminent law professor Alan Dershowitz accused the judge in the Donald Trump hush-money trial of being a thin-skinned tyrant and excoriated him for a threat that he said would deprive the former president of a fair trial.

In a column Tuesday in the New York Post, Mr. Dershowitz said he was left inside the court the previous day when New York Supreme Court Justice Juan Merchan cleared the court to speak with a defense witness.

“I have never seen a spectacle” like what transpired next, Mr. Dershowitz wrote.

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The court had been cleared of spectators and media after attorney Robert Costello, testifying in Mr. Trump’s defense, raised his eyebrows at a ruling by Justice Merchan.

“The court went berserk” in response, the law professor wrote.

“I observed one of the most remarkable wrong-headed biases I have ever seen. The judge actually threatened to strike all of Costello’s testimony if he raised his eyebrows again,” Mr. Dershowitz said.

The law professor, who has argued constitutional cases before the U.S. Supreme Court, called such a move would be obviously unconstitutional “because it would have denied the defendant his Sixth Amendment right to confront witnesses and to raise a defense.”

“It would have punished the defendant for something a witness was accused of doing,” he explained, calling the threat “absolutely outrageous, unethical, unlawful and petty.”

Mr. Dershowitz noted at the start of his column that he has seen and/or participated in trials in nearly 40 states, such foreign democracies as England and Israel and such foreign tyrannies as China and Russia.

“But in my 60 years as a lawyer and law professor, I have never seen a spectacle such as the one I observed sitting in the front row of the courthouse yesterday,” he wrote.

But he also said the threat was in keeping with how Justice Merchan has been biased against Mr. Trump.

“The judge in Donald Trump’s trial was an absolute tyrant, though he appeared to the jury to be a benevolent despot,” Mr. Dershowitz wrote, going on to call the courthouse clearing a case of the judge “losing his cool and showing his thin skin.”

Mr. Dershowitz said the behavior is an example of why courtrooms proceedings should be televised, a position he has held for decades. Television would show judges in action without the filter of a biased media.

“The public should have been able to see the judge in action, but because the case is not being televised, the public has to rely on the biased reporting of partisan journalists. But the public was even denied the opportunity to hear from journalists who saw the judge in action because he cleared the courtroom. I am one of the few witnesses to his improper conduct who remained behind to observe his deep failings,” the Harvard professor and longtime champion of due process wrote.