Australian journalist Peter Greste – a press freedom advocate and one of three Al Jazeera journalists formerly imprisoned in Egypt – has thrown his support behind a petition calling for the release of Australian filmmaker James Ricketson from Cambodian prison.

Ricketson was arrested in June on charges of “espionage” after he was spotted flying a drone at an opposition rally. He has spent more than six months in Phnom Penh’s overcrowded Prey Sar prison.

On December 31, Greste tweeted to his 50,000 followers: “Help free another journalist in prison on national security charges. No evidence that James Ricketson in Cambodia is guilty of anything other than caring.”

In 2013 and 2014, Greste was jailed for more than 400 days alongside his colleagues Baher Mohamed and Mohamed Fahmy after they were found guilty of “aiding a terrorist organisation” in Egypt, in a case that sparked an international outcry.

Greste said yesterday that elements of Ricketson’s case paralleled his own, and were part of a “very disturbing trend worldwide”.

“In the absence of any clear evidence . . . against James, it seems hard to escape the conclusion that this is intended to send a message to journalists and other critics of the government to shut up, to stop being critical, to stop supporting the opposition,” he said.

“Our experience in Egypt was incredibly infuriating because the judicial system was so politicised . . . I would urge the Cambodian authorities to make sure they do everything to operate openly and transparently and fairly.”

Last month the Committee to Protect Journalists reported 262 journalists were imprisoned worldwide, with Cambodia making the list after authorities arrested two former Radio Free Asia journalists in November, also on allegations of “espionage”.

“Using anti-state charges in terrorism or espionage legislation to silence critics is a really serious attack on democracy and freedom of speech,” Greste said.

The petition – which calls “shame” on Australian Foreign Minister Julie Bishop for apparent inaction in Ricketson’s case – claims he is one of 27 men in a single cell, which Prison’s Department spokesman San Keo yesterday said was entirely possible.

“I acknowledge that at Prey Sar the building was built for just 1,800 people, but now, there are up to 5,700 people there. So you can imagine how tight it is,” Keo said.

The Australian Department of Foreign Affairs and Trade reiterated it was offering consular assistance to a man detained in Cambodia, but would not respond to the allegation that Australia had a spy on Cambodian shores, or on whether Foreign Minister Bishop had appealed to her Cambodian counterpart for Ricketson’s release.

Ricketson’s petition had around 3,500 signatures as of December 30, a number that has now swelled to almost 6,000.

While many have championed Ricketson's work, he is a divisive figure, having been involved in public spats with Screen Australia and at times espousing conspiracy theories on blog. In 2014, he was handed a two-year suspended sentence for threatening to defame a Brisbane-based church by suggesting the church sold children. In late 2016, he was found guilty of defaming child protection NGO APLE by Cambodian courts, and the Cambodian Children's Fund has also accused him of harassment for what they characterise as "false allegations".