“I was tasked by Mr Trump to increase the total assets based upon a number he arbitrarily elected,” the 57-year-old Cohen told a packed Manhattan courtroom as Trump sat across from him at the defence table, scowling occasionally and shaking his head.

Cohen acknowledged responsibility for taking part in the alleged fraud and said he and former Trump Organization chief financial officer Allen Weisselberg would “reverse-engineer asset classes” to achieve “whatever number (Trump) told us”.

Trump would “look at the total assets and say, ‘I’m actually not worth US$4.5 billion. I am more like US$6 billion’,” Cohen said.

The 77-year-old Trump, the front runner for the 2024 Republican presidential nomination, denounced Cohen at the end of the day’s proceedings as a “disgraced felon”.

“The witness is totally discredited,” Trump said. “This is a case that should never have been brought.”

Cohen was sentenced in 2018 to three years in prison for campaign finance violations, tax evasion and participating in a hush money case involving Trump, but was released after a little over a year.

Trump’s lawyers sought to undermine Cohen’s credibility during cross-examination with one lawyer, Chris Kise, calling him a “serial liar”.

“You understand what under oath means?” another Trump lawyer, Alina Habba, sarcastically asked Cohen, who has been stripped of his law license.

It was Cohen’s testimony before Congress in 2019 that sparked the investigation by the New York authorities into whether Trump had artificially inflated his net worth.

Trump is not required to attend the trial, but he has shown up sporadically, using his appearances to portray himself as the victim of a supposed Democratic plot to interfere with his White House campaign.

The former president does not risk going to jail in the fraud trial, but New York Attorney General Letitia James, a Democrat, is seeking US$250 million in penalties and the removal of Trump and his sons from management of the family real estate empire.

Cohen is also expected to be a star witness in a separate criminal case facing the former president in New York – for allegedly paying election-eve hush money to a porn actress.

Earlier this month, Trump withdrew a lawsuit he filed against Cohen seeking US$500 million for alleged breach of lawyer-client privilege and a confidentiality agreement.

No reason was given for Trump dropping the suit, but Cohen noted that it came just days before the former president was to sit for a deposition.

The civil fraud and hush money cases are only two of the legal battles facing Trump as he seeks to recapture the White House.

He is to go on trial in Washington in March for conspiring to overturn the results of the 2020 election and in Florida in May on charges of mishandling secret government documents.

The twice-impeached former president also faces racketeering charges in Georgia for allegedly conspiring to upend the election results in the southern state after his 2020 defeat.

Jenna Ellis, in a tearful statement, expressed regret.

“I look back on this whole experience with deep remorse,” Ellis told the court after pleading guilty to a charge relating to false claims of voter fraud.

“If I knew then what I know now, I would have declined to represent Donald Trump in these post-election challenges,” she said.