Former US president Donald Trump was in a New York courtroom on Monday for the start of a civil fraud trial that could decide the fate of his flagship New York properties, including the Trump Tower, and limit his family's ability to do business in the city.

Trump was not required to be in the courtroom for the nonjury trial, but said he attended so he could "watch this witch hunt myself" and that he will return over the course of the proceedings.

"I'll be seeing a lot of you," he told reporters. "Because this is a horrible thing that's happening to this country." He called the civil trial the "single greatest witch hunt of all time".

In comments to reporters before and after leaving the courtroom, Trump railed against presiding state Supreme Court Judge Arthur Engoron, who ruled last week that Trump and his co-defendants were liable for fraud for overvaluing their properties.

He also attacked New York State Attorney General Letitia James, who filed the lawsuit accusing Trump of fraudulently inflating his assets by billions of dollars to secure favorable loan terms from banks.

The current nonjury trial concerns six remaining claims in the lawsuit, including allegations of conspiracy, insurance fraud and falsifying business records.

Trump has denied all wrongdoing, and his attorneys have described him as a "master of finding value where others do not", arguing that his alleged inflated valuations were a product of his business skills.

'Overvalued' assets

If the judge's initial ruling of last week is upheld on appeal, it could force Trump to give up New York properties such as Trump Tower, a mixed-use skyscraper on Fifth Avenue, a Wall Street office building, golf courses, and a suburban estate.

James is seeking to recover $250 million in alleged ill-gotten gains. She wants to prohibit Trump and the Trump Organization from entering any New York state commercial real estate deals for the next five years and to bar them from applying for loans from any New York bank during that same period.

At the end of the trial's first day, Trump left claiming he had scored a victory, pointing to comments that he viewed as the judge coming around to the defense view that most of the suit's allegations are too old.

The judge suggested that testimony about Trump's 2011 financial statement was beyond the legal time limit.

Trump's campaign immediately began fundraising off his appearance.

After the fiery first day of opening arguments, lawyers in Trump's business fraud trial continued on Tuesday to the more plodding task of going through years of his financial documents.

Agencies contributed to this story.