A defamation lawsuit launched by Lachlan Murdoch, the eldest son of media mogul Rupert Murdoch, against Australian news website Crikey had its first day in court on Friday.

Mr Murdoch, who is the Fox Corp chief executive and News Corp co-chairman, is suing Private Media over an opinion piece about the storming of the US Capitol.

He claims he was defamed in the article by Crikey’s political editor, Bernard Keane, which was headlined: “Trump is a confirmed unhinged traitor. And Murdoch is his unindicted co-conspirator.”

The first day of proceedings was taken up by legal argument over the terms of the defence and the case will return to court on October 10.

In its defence, filed with the court on Friday, Crikey argued the opinion piece was in the public interest.

Last month, Crikey posted a full-page advertisement in The New York Times that invited the lawsuit, saying it wanted to “test this important issue of freedom of public interest journalism in a courtroom”.

The defamation lawsuit was filed the following day.

The website has framed the battle as one of free speech and highlighted differences in size, and wealth between itself and the companies owned by the Murdoch family.

It has launched a Go Fund Me for its defence and, in a statement released on Thursday, chief executive Will Hayward said: “We think it is important in an open, well-functioning society that the rich and powerful can be critiqued.”

In a statement of claim, a lawyer representing Mr Murdoch accused Crikey of using the situation for “self-promotion” and to “publicise the Crikey website and increase subscribers” and said the claim of public interest journalism was “disingenuous”.