Donald Trump’s son Donald Trump Jr has testified that he had little to do with financial practices at the heart of a New York civil fraud trial accusing the former US president and his family businesses of inflating asset values to dupe lenders and insurers.

Donald Jr, an executive vice president at the Trump Organisation and a co-defendant in the case, is the first of Trump’s adult children to testify, followed by Eric and Ivanka Trump. Their father is set to testify on Monday.

Wearing a blue suit and pink tie, Donald Jr on Wednesday said he and Eric largely took over management of the company when their father assumed the presidency in 2017.

But he said had little involvement in financial matters that are at the heart of the case. His testimony could continue into Thursday. It could be another flashpoint in a trial punctuated by sharp exchanges between lawyers and witnesses and heated arguments over the admissibility of evidence.

The lawsuit by Democratic New York Attorney-General Letitia James accuses Trump, his two adult sons and a handful of their family businesses of inflating their assets by billions of dollars to secure better loan terms.

The trial is one of many legal troubles Trump faces as he campaigns to retake the White House. He holds a commanding lead over his rivals for the Republican presidential nomination to face Democratic President Joe Biden in the November 2024 election, opinion polls show.

The elder Trump also faces four separate criminal prosecutions, including cases in Washington and Georgia related to his attempts to overturn his 2020 election defeat.

Trump has denied wrongdoing and has repeatedly accused James and Judge Arthur Engoron of political bias.

“Leave my children alone, Engoron. You are a disgrace to the legal profession!” he wrote on social media on Wednesday morning.

Trump is under a limited gag order barring him from speaking publicly about court staff but has twice violated it and been fined $US15,000 ($A23,500) for attacking Engoron’s top clerk, whom he has accused of bias. He has not faced sanctions for his attacks on Engoron or James.

The Washington judge overseeing his federal election subversion trial has also imposed a limited gag order. Trump has alleged that both violate his right to free speech.

Trump himself is not required to attend trial, and did not appear in court on Wednesday.

He appeared in court last week for a dramatic reunion with his former lawyer and fixer Michael Cohen, who testified against his former boss last week.

Cohen, appearing face-to-face with Trump for the first time since their acrimonious break five years ago, said Trump directed him to inflate asset values to arrive at the arbitrary net worth he desired.

Trump has a campaign event scheduled in Houston on Thursday, when Eric Trump is set to testify.

Before the trial began, Engoron found in September that Trump fraudulently inflated his net worth and ordered the dissolution of companies that control pillars of his real estate portfolio, including Trump Tower in Manhattan. That ruling is on hold while Trump appeals.

The trial largely concerns damages. James is seeking at least $US250 million ($A391 million) in fines, a permanent ban against Trump and his two adult sons from running businesses in New York and a five-year commercial real estate ban against Trump and the Trump Organisation.

– AAP