Courtroom tensions were high as porn star Stormy Daniels made her keenly anticipated debut in the witness box, giving a graphic account of her alleged sexual encounter with Donald Trump.

The details were so salacious that Trump’s lawyer unsuccessfully sought a mistrial, while even Judge Merchan appeared frustrated at the extent of Daniels’ testimony.

“The degree of detail that we’re going into here is just unnecessary,” he said, also chastising the prosecution for soliciting too much information.

Daniels, 45, is at the centre of the first criminal trial of a former US president over claims he paid the adult entertainer hush money to prevent her from sharing her story when he was running for election.

Prosecutors say Trump, 77, covered up a $US130,000 ($196,745) payment his former lawyer Michael Cohen made to Daniels in the run-up to the 2016 US presidential election.

Cohen has previously said Trump directed him to pay Daniels to keep quiet about a sexual encounter she says she had with Trump in 2006.

Daniels, whose real name is Stephanie Clifford, took the stand wearing all black, with black-framed glasses.

She spoke about being introduced to Trump at a golf tournament at Lake Tahoe in 2006, during a meet and greet with adult actresses and golfers.

Trump took note when an associate mentioned Daniels directed films.

“He said, ‘Oh, you direct too – you must be the smart one’,” Daniels testified.

Daniels told jurors that later in the day, Trump’s bodyguard approached her and said Trump would like her to join him for dinner.

“F no,” Daniels said when asked how she initially responded.

She said she changed her mind after a publicist convinced her the dinner could make a great story. When she arrived at his hotel suite, Trump greeted her wearing only satin pyjamas.

“I said, ‘Does Hugh Hefner know you stole his pyjamas?’,” Daniels recalled saying, referring to Playboy impresario Hugh Hefner.

Daniels told Trump to change, and he politely obliged, she said, returning in pants and a shirt.

Stormy Daniels says she had brief sex with Donald Trump. Photo: Getty

The alleged encounter was a year after Trump married wife Melania in 2005.

Trump denies any sexual encounter with Daniels.

“Oh, ‘what about your wife?’,” Daniels said she asked Trump. “He said, ‘Don’t worry about that, we don’t even sleep in the same room’.”

Daniels said she went to the toilet and returned to discover Trump on the bed wearing only boxer shorts and a T-shirt.

“I thought, ‘Oh my God, what did I misread to get here?’,” she said.

“At first I was just startled, like a jump scare. I wasn’t expecting someone to be there, especially minus a lot of clothing.”

Daniels said Trump stood between her and the door, but “not in a threatening manner”.

She said “the intent was pretty clear”.

“There was an imbalance of power for sure. He was bigger and blocking the way. I was not threatened verbally or physically”.

Daniels alleged they had “brief” sex in the missionary position and Trump did not wear a condom.

Stormy Daniels in the witness box. Picture: AAP

While listening to Daniels’ testimony, Trump leaned back in his chair at the defence table and sometimes appeared to close his eyes.

Before her testimony, Merchan said Daniels would be allowed to speak about the basic details of the encounter. Defence lawyer Susan Necheles objected, arguing that the evidence was peripheral to a case centred on financial records.

Prosecutor Susan Hoffinger said the testimony was needed to complete the story and establish Daniels’ credibility.

“In terms of the sexual act, it will be just very basic. It’s not going to involve descriptions of genitalia or anything of that nature,” Hoffinger said.

Trump, who is running again for president in the November 5 election, has pleaded not guilty to 34 felony counts of falsifying business records.

Prosecutors have shown the former president’s signature was on payments at the heart of the case.

They say Trump falsely labelled his reimbursement payments to Cohen in 2017 as legal expenses in his real estate company’s books to cover up what they call an illegal scheme to influence the 2016 election by buying the silence of people with potentially damaging information.

Donald Trump was in court when Stormy Daniels testified. Photo: Getty

Trump’s lawyers have suggested she was seeking a role on Trump’s reality TV show The Apprentice.

Daniels has been at the receiving end of some of Trump’s vitriolic attacks on social media.

Merchan, who is hearing the case, has said some of those posts violated a gag order restricting Trump from speaking about witnesses, jurors, and others involved in the case if those statements are meant to influence the proceedings.

Trump has been fined $US10,000 so far for violating gag orders that prevent him from talking about witnesses.

Merchan has warned Trump could be jailed if he keeps up his public criticism.

Trump, the Republican presidential candidate has called the order a violation of his free speech rights and says the trial is an attempt to hobble his attempt to win back the White House from Democratic President Joe Biden.

-with AAP