Donald Trump's estranged former fixer Michael Cohen began testifying on Monday at the former U.S. president's criminal trial, where he was expected to tell jurors 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.

He started his testimony by explaining that he went to law school at his grandmother's urging, even though he "really didn't want to be a lawyer."

In 2007, he left his job at a law firm to join Trump's New York-based family real estate company. Trump offered him a job, Cohen said, after Cohen presented Trump with a $100,000 bill for work his firm had done for one of Trump's companies.

"I was honored. I was taken by surprise, and I agreed," Cohen said, adding that Trump never paid the bill.

Asked to identify Trump, Cohen looked to his left and stood up.

"He's wearing a blue and white tie," Cohen said. Trump, his eyes closed, did not react.

For nearly a decade, Cohen, 57, worked as an executive and lawyer for Trump's 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 Nov. 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 on podcasts.

While the jurors saw Cohen in person for the first time, his presence has loomed over the trial. Witnesses have spoken about him dozens of times, while Trump's defense lawyers attacked his credibility from the trial's outset, calling him an untrustworthy liar in their opening statement.

On Friday, Justice Juan Merchan urged prosecutors to tell Cohen to stop making public statements about the case after defense lawyer Todd Blanche said Cohen had posted on social media while wearing a T-shirt showing Trump behind bars.

Cohen's $130,000 hush money payment to the porn star Stormy Daniels before the 2016 election, intended to keep her from speaking publicly about a 2006 sexual encounter she says she had with Trump, is at the center of the trial.

Prosecutors with Manhattan District Attorney Alvin Bragg's office have accused Trump of falsely labeling his reimbursement payments to Cohen in 2017 as legal expenses in his New York-based real estate company's books.

They say the altered business records covered up election-law and tax-law violations that elevate the 34 counts Trump faces from misdemeanors to felonies punishable by up to four years in prison.

Trump has pleaded not guilty to all 34 counts and denies having had a sexual encounter with Daniels. He argues the case is a politically motivated attempt to interfere with his campaign. Bragg, the prosecutor, is a Democrat.

"Fat Alvin, corrupt guy," Trump said of Bragg at a New Jersey political rally on Saturday night.

Trump arrived at the courthouse on Monday with several Republican lawmakers in tow, including U.S. senators J.D. Vance and Tommy Tuberville and U.S. Representative Nicole Malliotakis.

Prosecutors say the payment to Daniels was part of an illegal scheme to influence the 2016 election by buying the silence of people with potentially damaging information. Trump's lawyers say it was to spare him embarrassment with his family.

Cohen pleaded guilty in 2018 to violating federal campaign finance law by paying off Daniels and testified that Trump directed him to make the payment. Federal prosecutors did not charge Trump with any crime.

Trump's defense lawyers have told the 12 jurors and six alternates that Cohen is a liar who acted on his own when paying Daniels, seeking to distance Trump from the reimbursement checks and invoices at the heart of the case.

Cohen has admitted to lying under oath multiple times, providing substantial fodder for the defense to undermine his credibility.

He has acknowledged lying to the U.S. Congress in 2017 about a Trump Organization real estate project in Moscow, but has since said he did so to protect Trump.

He also pleaded guilty to violating tax law in 2018, but now claims he did not commit that crime.

COHEN A TARGET OF TRUMP ANGER

During testimony last week jurors saw the 34 invoices, corporate ledger entries and checks that prosecutors say were falsified by Trump to obscure his reimbursement to Cohen.

A former Trump employee testified he had been told by Trump's top financial officer that the reimbursements to Cohen were for expenses incurred during the campaign. That could undercut an argument made by Trump's lawyers that the payments were for legal work.

However, neither that employee nor another who testified last week was able to say whether Trump himself directed the falsification of the records to hide the payment to Daniels - a hole that prosecutors will aim to fill with Cohen's testimony.

Cohen has been on the receiving end of Trump’s vitriolic social media attacks, some of which the judge has said violated a gag order restricting Trump from making statements about witnesses, jurors and families of the judge and prosecutors if meant to interfere with the case.

Trump has called the gag order a violation of his right to free speech, and has said it is unfair to bar him from responding to attacks by witnesses such as Cohen and Daniels.

Merchan has fined Trump $10,000 for 10 violations of the order and warned the former president he could face time in jail if he keeps up his attacks.

The case is widely seen as less consequential than three other criminal prosecutions Trump faces, but it is the only one certain to go to trial before the election.

The other cases charge Trump with trying to overturn his 2020 presidential defeat and mishandling classified documents after leaving office. Trump pleaded not guilty to all three.