Stormy Daniels, the porn star at the heart of Donald Trump's hush money case, took the witness stand on Tuesday at the blockbuster trial of the former US president.

"The people call Stormy Daniels," prosecutor Susan Hoffinger announced as the 77-year-old Trump sat at the defence table in the Manhattan courtroom flanked by his lawyers.

Daniels' testimony is expected to be a pivotal moment in the courtroom drama rocking the scandal-plagued Republican's attempt to recapture the White House.

Daniels, 45, a long-time actor and director in the adult film industry, says she had a sexual encounter with Trump in 2006 and was then paid to keep quiet about it on the eve of his 2016 election against Hillary Clinton, when the news could have sunk his campaign.

Trump denies ever having sex with Daniels and Susan Necheles, a lawyer for the former president, lodged an objection with the judge presiding over the trial to allowing her to testify about any "sexual acts."

"It is unduly prejudicial," Necheles told Judge Juan Merchan.

Hoffinger said Daniels' testimony would not be graphic and would "omit details that are too salacious."

Prosecutors say that Trump – desperate to kill Daniels' story before it could wreck his chances in the narrow 2016 race – illegally reimbursed his fixer and lawyer, Michael Cohen, to cover up the US$130,000 hush money payment.

He is charged with 34 felony counts of falsifying business records.

Cohen, who has become a vocal critic of his former boss, is also expected to testify at the trial as a prosecution witness. (AFP)