The controversial trial of staff from Turkey's main opposition newspaper resumed on Monday in a case seen as a test for press freedom under President Recep Tayyip Erdogan.

The case, which opened in Istanbul in July, involves 17 current and former writers, cartoonists and executives from Cumhuriyet ("Republic") who are being tried on "terror" charges in a move denounced by supporters as absurd.

For government critics, the case is emblematic of the erosion of freedom following last year's failed coup when Ankara launched a massive crackdown targeting those with alleged links to the putschists as well as opponents.

The secular daily is one of the few voices in the Turkish media to oppose Erdogan, with its embarrassing scoops angering those in the halls of power.

On July 28, an Istanbul court freed seven of the newspaper's staff after 271 days, including respected cartoonist Musa Kart and Turhan Gunay, editor of the books supplement.

But some of the paper's most prominent staff remain in custody, among them commentator Kadri Gursel, investigative journalist Ahmet Sik, editor-in-chief Murat Sabuncu and chief executive Akin Atalay.

Eight other suspects have also been charged but are not being held in prison.

Sik has been behind bars for 255 days while the other three have been jailed for 316 days. If convicted, they face varying terms of up to 43 years in jail.

Sik is the author of an explosive 2011 book entitled "The Imam's Army", which exposed how followers of influential Muslim preacher Fethullah Gulen infiltrated the Turkish bureaucracy and built ties with the ruling party.

Once a close ally of Erdogan, Gulen is now in self-imposed exile in the United States and is wanted on charges of ordering the failed July 2016 coup, with Ankara arresting more than 50,000 people on suspicion of links to his movement. He denies the charges.

- 'Shameful trial' -

The second session of hearings is taking place in an underground court next to the high-security Silivri prison on the outskirts of Istanbul where the defendants are being held.

Those on trial are charged with using their position to support the Kurdistan Workers' Party (PKK), the ultra-left Revolutionary People's Liberation Party-Front (DHKP-C), and the Gulen movement.

Ankara has branded all three terror organisations.

The daily's imprisoned accountant, Yusuf Emre Iper, insisted he was innocent of the allegations that he had downloaded the Bylock messaging app which authorities suspect was used by Gulen's supporters to coordinate the coup bid.

Iper told the court he was "sure" he would be released quickly after his detention in April and that he begged the prosecutor "to examine my phone" at the time.

"I ask for my acquittal in this shameful trial," Iper added, denying any links with the Gulen movement.

The judge asked several witnesses including Cumhuriyet journalists and former members of the foundation which owns the daily, about its financial situation and the editorial process: how headlines are chosen and the angle of stories.

Christophe Deloire, Reporters Without Borders (RSF) secretary general, said the journalists "are judged simply because they embody the journalism worthy of its name in Turkey and they do not broadcast the propoganda of the Erdogan regime".

"We must not abandon Turkish journalists," he said outside the court.

- 'We want justice' -

Also on trial, but in absentia, is the paper's former editor-in-chief Can Dundar, who was last year sentenced to five years and 10 months in jail over a front-page story accusing the government of sending weapons to Syria.

Dundar has now fled Turkey for Germany.

In the indictment, the newspaper was accused of an "intense perception operation" targeting both Turkey and Erdogan using the tactics of an "asymmetric war".

The paper's supporters claim the charges amount to "punishment", insisting Cumhuriyet has always been strongly opposed to the three groups.

On the daily's front page on Monday, the headline read "We want justice" with images of those still imprisoned.

According to the P24 press freedom group, there are 170 journalists behind bars in Turkey, most of whom were arrested after the coup in a move that has alarmed Turkey's Western allies.

Turkey ranks 155 out of 180 on the latest RSF world press freedom index.