Donald Trump's estranged former fixer Michael Cohen took the witness stand on Monday at the former U.S. president's criminal trial, where he was expected to testify that he helped Trump illegally hide a payment to silence a porn star who said they had a sexual encounter.

Once one of Trump's most loyal lieutenants, Cohen is the prosecution's star witness as the trial enters its fifth week in New York state criminal court in Manhattan.

Trump did not visibly react as Cohen took the stand.

For nearly a decade, Cohen, 57, worked as an executive and lawyer at Trump's New York-based family real estate company and once said he would take a bullet for Trump, a Republican trying to take back the White House from Democratic President Joe Biden in this year's November 5 U.S. election.

Cohen, who served as Trump's personal lawyer after his presidential term began in 2017, broke with him when federal prosecutors probing Trump's 2016 presidential campaign focused on Cohen. 

He has become one of Trump's most outspoken critics, frequently disparaging him on social media and podcasts.