Human rights activists, including Amnesty International head, go on trial in Turkey
Police blocked off the square outside the court, Istanbul’s Palace of Justice in the central Caglayan district, although the area was thronged with journalists.
All except two of the activists, including two foreigners, have been imprisoned since a police raid in July on a workshop run by Amnesty International on the Buyukada island off Istanbul.
The accused include the director of Amnesty Turkey, Idil Eser, who was detained during the raid and Amnesty's Turkey chair Taner Kilic, who was arrested in June.
Kilic has been charged with membership of an armed terror group while the others are charged with "aiding" an armed terror group.
The activists face up to 15 years in jail if convicted after the trial at Istanbul's main court. They are accused of seeking to create "chaos in society" similar to the anti-government protests which rocked Turkey in 2013.
The charges amplify concerns over freedom of expression in Turkey under the state of emergency imposed following last year's failed coup.
Over 50,000 people have been arrested following the failed July 2016 coup, including journalists and politicians.
More than 140 media organisations and nearly 2,000 NGOs have been shuttered.
Turkish President Recep Tayyip 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.
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.
Gardner said the "scandalous case ... must be finally put to bed" three months later and that the human rights defenders must be released "unconditionally".