Supporters of Julian Assange have written to new US ambassador to Australia Caroline Kennedy calling for the release of the WikiLeaks founder.

Advocates for the 50-year-old Australian journalist have gathered outside Parliament House in Canberra, in the wake of his appeal to the High Court in London to block his extradition to the US to face criminal charges.

It’s the latest step of a legal battle that has dragged on for more than a decade.

Filmmaker James Ricketson, who spent 15 months in a Cambodian prison over espionage charges before being pardoned, has written a letter to Ms Kennedy quoting her late father, the former US president.

“The very word ‘secrecy’ is repugnant in a free and open society,” John F. Kennedy said in April 1961.

“No president should fear public scrutiny of his program.

“For from that scrutiny comes understanding, and from that understanding comes support or opposition. And both are necessary.”

Mr Ricketson said in the letter the words applied just as much today as they did at the height of the Cold War.

“Julian Assange has applied the precepts outlined here by your father, and reiterated by President Biden,” he wrote to Ms Kennedy.

“Please remind your president, and those in the Biden administration that you serve, of your father’s words.”

Assange is wanted by US authorities on 18 counts, including a spying charge, relating to WikiLeaks’ release of vast troves of confidential US military records and diplomatic cables. US officials claim the leak put lives in danger.

UK Home Secretary Priti Patel, who approved his extradition, said British courts had concluded his extradition would not be incompatible with his human rights, and that he would be treated appropriately.

Independent MP Andrew Wilkie, a former intelligence analyst, said the extradition of Assange would be a “direct attack on press freedom and set the frightening precedent for all Australians, particularly for journalists”.

“I have no doubt Mr Albanese has enough influence and good relationships to pick up the phone and end this madness. It’s beyond time for the government to say enough is enough and to bring Julian home.”

Mr Albanese has said he didn’t see the purpose of the “ongoing pursuit” of Assange, but he insists the government will deal with the matter through diplomatic channels.