Novelist Asli Erdogan has been held in jail for 132 days since her detention over her links to a pro-Kurdish newspaper the authorities regard as a mouthpiece for the outlawed Kurdistan Workers Party (PKK).
Also on trial is Necmiye Alpay, an internationally prominent linguist who has also made widely praised translations of Western novels into Turkish. She has been held behind bars for 120 days.
Including Erdogan, 49, and Alpay, 70, a total of nine suspects linked to Ozgur Gundem have been charged in the case.
Erdogan is no relation to Turkish President Recep Tayyip Erdogan -- the name is common in Turkey. There had been reports in November the pair were being released but these were swiftly denied.
She has published several well-received novels including "The City in Crimson Cloak", which was also translated into English.
"They are being prosecuted for writings and thoughts that are in no way reprehensible according to Turkish law," said Baris Yarkadas, Istanbul MP for the opposition Republican People's Party (CHP).
"We are here not just to show solidarity with the accused but also to defend democracy," said Murat, 48.
"It's now or never, otherwise it's too late and dictatorship is there."
Activists worry about a drastically worsening climate for journalists in Turkey, in particular since the state of emergency imposed in the wake of the July 15 coup.
According to the P24 Platform for Independent Journalism, 118 journalists have been arrested during the state of emergency, 80 of them within the coup probe.
Turkish authorities today detained prize-winning journalist Ahmet Sik over a succession of tweets and articles for an opposition daily.
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