New York, New York - Donald Trump's one-time fixer and the star prosecution witness in the former president’s historic criminal trial testified Monday in a dramatic courtroom face-off with his former boss.

Michael Cohen's testimony followed last week's court proceedings in which adult film star Stormy Daniels gave in-depth details about her alleged sexual encounter with Trump which is at the heart of the case.

Trump is accused of falsifying business records to reimburse his lawyer Cohen for a hush-money payment to Daniels on the eve of the 2016 presidential election, when the story could have proved politically fatal.

Cohen spent just over 13 months in prison and a year and a half under house arrest after he was sentenced to three years imprisonment for lying to Congress and financial crimes.

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In the first weeks of the trial, jurors heard from witnesses that Cohen was a difficult character who bullied and cajoled others to get his way, while the defense counsel painted him as a pathological liar and convicted criminal.

A source close to Cohen told AFP ahead of his appearance that "everything he says is backed up by documents. He has a high bar to prove his sincerity – but he's proven he can."

Daniels, who claims to have had sex with Trump in 2006, denied she threatened him if he did not buy her silence for $130,000, a payment that prosecutors say Trump then covered up.

The trial is taking place six months before the November election when Trump will try to defeat Democratic President Joe Biden.

During nearly eight hours over two days last week, Daniels walked the New York jury through the one-night stand she said she had with Trump at a celebrity golf tournament, and then the financial settlement she says ensued.

Trump spoke at a rally on a beach in New Jersey on Saturday. In his speech, he condemned the judge as "conflicted" and the prosecutor in the case Alvin Bragg as a "radical Democrat."

But he appeared to heed the gag order and avoided naming any witnesses in the case, including Cohen.