Cumhuriyet journalists were arrested after police found they communicated with users of the Bylock app, linked to coup plotters
Seventeen journalists from Turkey's opposition Cumhuriyet newspaper denied claims that they were supporting a terrorist group and pleaded not guilty in a Turkish court on Monday.
Turkish prosecutors accused the reporters who have spent more than eight months in prison of waging "asymmetric war", a charge dubbed by the detained journalists as "illogical".
The case has been seen by government critics and activists around the world as a further sign that freedom of expression inside Turkey being under attack.
"Journalism is not a crime," chanted several hundred people gathered outside the central Istanbul court to protest against the prosecution of writers, executives and lawyers of the staunchly secularist Cumhuriyet newspaper.
The hearing coincides with an escalating dispute with Germany over the arrest in Turkey of 10 rights activists, including one German, as part of a crackdown since last year's attempted coup against President Tayyip Erdogan.
Turkish prosecutors are seeking up to 43 years in jail for staff from the paper, including some of Turkey's best-known journalists, who are accused of targeting Erdogan through "asymmetric war methods".
"According to the government, everyone in opposition is a terrorist, the only non-terrorists are themselves," Filiz Kerestecioglu, a member of parliament from the pro-Kurdish HDP opposition party, told reporters ahead of the trial.
A man holds a portrait of jailed journalist Kadri Gursel on 24 July 2017 during a demonstration outside Istanbul's courthouse (AFP)
In the 324-page indictment, Cumhuriyet was effectively taken over by the network of US-based cleric Fethullah Gulen, blamed for the failed putsch last July, and used to "veil the actions of terrorist groups".
Gulen has denied any involvement in the coup.
The newspaper has called the charges "imaginary accusations and slander".
Social media posts comprised the bulk of evidence in the indictment, along with allegations that staff had been in contact with users of Bylock, an encrypted messaging app the government says was used by Gulen's followers.
Rights groups and Turkey's Western allies have complained of deteriorating human rights under Erdogan.
In the crackdown since last July's failed coup, 50,000 people have been jailed pending trial and some 150,000 detained or dismissed from their jobs.
As part of the purge, some 150 media outlets have been shut down and around 160 journalists are in jail, according to the Turkish Journalists' Association.
A man holds portrait of a jailed journalist as a woman sits during a demonstration in front of Istanbul's courthouse on 24 July 2017 (AFP)
The crackdown has strained Turkey's ties with the European Union, but the reaction from the bloc has been restrained because it depends on Turkey to kerb the flow of migrants and refugees into Europe.
However, Europe's leading power, Germany, has stepped up the pressure in recent days, threatening measures that could hinder German investment in Turkey and review Turkish applications for arms deals.
Turkish authorities say the crackdown is justified by the gravity of the coup attempt, in which rogue soldiers tried to overthrow the government and Erdogan, killing 250 people, most of them civilians.
Cumhuriyet is accused of writing stories that serve "separatist manipulation".
The newspaper's editor Murat Sabuncu and other senior staff have been in pre-trial detention since being arrested in November.
Other defendants include well-known columnist Kadri Gursel and Ahmet Sik, who once wrote a book critical of Gulen's movement. Former editor Can Dundar, who is living in Germany, is being tried in absentia.
Cumhuriyet (Republic), which was set up in 1924 and is Turkey's oldest mainstream national title, has been a thorn in the side of Erdogan in recent years.
It is one of the few genuine opposition voices in the press, which is dominated by strongly pro-government media and bigger mainstream dailies that are increasingly wary of challenging the authorities.
Also being tried in the case is the investigative journalist Ahmet Sik who in 2011 wrote an explosive book "The Imam's Army" exposing the grip Gulen's movement had on the Turkish state.
Eleven of the 17 including Gursel, Sabuncu, Kart and Sik, are being held in custody. The first stage of the trial is expected to last until Friday.
Since their arrests, Cumhuriyet has continued publishing the columns of the jailed journalists but with a blank white space instead of text.
"This trial is a test for Turkey," Aydin Engin, one of the writers on trial who was freed after his initial arrest. "Erdogan says justice is balanced in Turkey. Now we will see."
Being tried 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.
He has now fled Turkey for Germany.