Eleven human rights activists, including the two top figures with Amnesty International in Turkey, are due to go on trial Wednesday on contested terror charges.
All bar two of the activists -- who include two foreigners -- have been behind bars since a police raid in July on a workshop run by Amnesty on a popular island off Istanbul.
They face up to 15 years in jail if convicted after the trial at Istanbul's main court.
The accused include the director of Amnesty Turkey Idil Eser, who was detained in the raid by police on the workshop on the island of Buyukada.
Also going on trial is Amnesty's Turkey chair Taner Kilic, who was detained in June and whose case has been merged with that of the other 10 activists as prosecutors claimed he was aware of preparations for the workshop.
He has been charged with membership of an armed terror group while the others are charged with "aiding" an armed terror group.
The activists are accused in the indictment of seeking to create "chaos in society", similar to the anti-government protests that rocked Turkey in the summer of 2013.
The charges amplified concerns over freedom of expression in Turkey under the state of emergency imposed after last year's failed coup aimed at unseating President Recep Tayyip Erdogan.
Ankara blamed the coup attempt on the self-exiled Muslim preacher Fethullah Gulen, who denies the accusations.
Erdogan in July said the activists were detained after a tip-off that they were working against the government, comparing them to those involved in the failed coup.
- 'Test case'-
Amnesty International's Turkey researcher Andrew Gardner said the case against the human rights defenders was "a completely baseless prosecution that doesn't have a shred of evidence and doesn't stand up to the slightest scrutiny".
"The case will be a test case for Turkey's judiciary," he told AFP.
The two foreigners -- German Peter Steudtner and Swede Ali Gharavi -- were leading a digital information workshop on Buyukada and have been under arrest since the July raid.
Steudtner's detention has stoked tensions in particular with Berlin, and German Foreign Minister Sigmar Gabriel has described the terror charges as "incomprehensible".
Sweden this month summoned Turkey's ambassador over the case of Gharavi, saying it was "worried" about the accusations against him.
Amnesty posted a video message from Edward Snowden -- a former CIA employee who in 2013 revealed spying activities of the National Security Agency -- in support of the detained rights activists.
"The director and the chair of Amnesty International in Turkey are in prison for defending human rights," Snowden said.
"I know what it means to have support from the outside world in that moment, when you are alone, when you need it most. Join me, and together let's stand up for human rights defenders in Turkey."
Amnesty supporters from 110 countries worldwide stood up for Snowden as part of a campaign. Thousands wrote solidarity messages as well.
The suspects in Turkey are accused of links to outlawed groups including the Kurdistan Workers' Party (PKK), Gulen's organisation and the far-left Revolutionary People's Liberation Party-Front (DHKP-C).
Gardner said the "scandalous case ... must be finally put to bed" three months later and that the human rights defenders must be released "unconditionally".