Two women went on trial on Wednesday over false claims that France's first lady Brigitte Macron was transgender, which sparked online rumour-mongering by conspiracy theorists and the far right.

In 2022, Brigitte Macron filed a complaint for libel against two women who posted a YouTube video in December 2021 alleging she had once been a man named "Jean-Michel".

The claim went viral just weeks before the 2022 presidential election.

The trial on defamation charges comes amid a frenetic campaign for snap legislative polls called by President Emmanuel Macron after the far right trounced his party in EU parliament elections.

Amandine Roy, a self-proclaimed spiritual medium, appeared in court in Paris on Wednesday to answer questions about the interview she conducted with Natacha Rey, an independent journalist who did not turn up citing illness.

Roy, 49, had interviewed Rey for four hours on her YouTube channel in which the journalist spoke about the "state lie" and "scam" that she claimed to have uncovered.

Rey was "desperate to share her work", said Roy, who had merely "acquiesced to her request".

As for the credibility of the claims, Roy insisted that Rey "had spent three years researching, it's not like she pulled it out of her hat".

"My regret is that this wasn't taken up and investigated by the mainstream media," said Roy, who said she could not "hide" such a "serious" subject.

Neither the president, 46, nor the 71-year-old first lady were in court.

Messages multiplied on social media claiming that the first lady, formerly Brigitte Trogneux, had never existed and that her brother Jean-Michel had changed gender and assumed that identity.

The false claim also led to more serious accusations of child abuse brought against France's first lady.

"The prejudice is massive, it exploded everywhere," said Brigitte Macron's lawyer, Jean Ennochi.

He demanded 10,000 euros ($10,750) in compensation for each of Brigitte Macron and her brother.

The disinformation even spread to the United States where Brigitte Macron was attacked in a now deleted YouTube video ahead of the November elections.

Brigitte Macron is among a group of influential women -- including former US first lady Michelle Obama and New Zealand ex-premier Jacinda Ardern -- who have fallen victim to the growing trend of disinformation about their gender or sexuality to mock or humiliate them.

A decision on the case is due to be made on September 12.