Donald Trump has complained of unfair treatment in a defiant performance on the witness stand, prompting the judge overseeing his New York civil business-fraud trial to threaten to cut his testimony short.

Under questioning about his company’s accounting practices, Trump repeatedly clashed with Judge Arthur Engoron, who is weighing whether to impose hundreds of millions of dollars in fines and other penalties that could hobble the real estate empire that vaulted Trump to prominence.

Engoron warned former US president Trump, the frontrunner for the Republican nomination in the 2024 election, that he might remove him from the witness stand if he did not answer questions directly.

“Mr Kise, can you control your client?” Engoron asked Trump’s lawyer, Christopher Kise. 

“This is not a political rally. This is a courtroom.”

Trump often avoided direct answers, instead bragging about his properties and his wealth. 

He accused New York Attorney General Letitia James, an elected Democrat, of targeting him in a lawsuit to further her political career and accused Engoron of lowballing the value of his properties.

“The fraud is on behalf of the court,” Trump said.

Both men repeatedly raised their voices as the judge sought to prevent Trump from using the court to air his grievances.

“I’m not here to hear what he has to say. I’m here to hear him answer questions. Sit down already,” he told Alina Habba, another one of Trump’s lawyers.

Amid the fireworks, Trump acknowledged that his company did not provide accurate estimates of the value of apartment towers, golf courses and other assets. 

New York state lawyers said those values were pumped up to win better financing terms, and Engoron has already ruled that they were fraudulent.

But Trump said many of them, such as his Mar-a-Lago estate and Doral golf course in Florida were undervalued, and that banks did not take the valuations seriously.

“It wasn’t important. You’ve made it important but it wasn’t,” Trump said of the estimates.

New York state lawyers said in their lawsuit that the estimates misled lenders and insurers, earning him $US100 million ($A154 million) and exaggerating his wealth by $US2 billion.

Trump has repeatedly called the case a “witch hunt” and has accused Engoron and New York Attorney General Letitia James of political bias.

He kept up those complaints on the witness stand, where he accused legal authorities of paying unduly close attention to his business after he won the 2016 presidential election.

“I’m sure the judge will rule against me because he always rules against me,” he said.

“This is a very unfair trial,” he added later.

At one point when Trump was on the stand, Engoron asked Kise to take Trump to the back of the courtroom and “explain the rules”.

“The former and again soon to be president of the United States understands the rules,” Kise responded.

Unlike the four criminal cases Trump faces, this civil trial does not threaten to put him in prison as he mounts a comeback White House bid.

Indeed, Trump has sought to take advantage of the legal cases, using them to solicit campaign donations and argue that he is being targeted for his political views.

James is seeking $US250 million in fines, as well as restrictions that would prevent Trump and his sons Eric and Donald Jr from doing business in their home state.

In testimony last week, both sons said they were unfamiliar with the details of the valuation documents. 

Trump made that argument as well, saying accountants did the bulk of the work.

Engoron has already cancelled business certificates for companies that control large portions of his business although that order is on hold during appeal.