Eleven human rights activists including the Turkish head of Amnesty International have gone on trial on charges of terrorism, a case that has become a flashpoint in Turkey's tensions with European allies.
Police blocked off the main square near the Istanbul courthouse on Wednesday, although journalists and supporters of the defendants thronged the area.
A group of around 50 people from human rights groups, foreign consulates and women's rights groups stood outside the court holding signs reading "Free Rights Defenders".
The activists, also including a German and a Swedish national, face up to 15 years in prison on charges that include membership in and aid to an "armed terrorist organisation".
They were detained in July after participating in a workshop on digital security held on an island near Istanbul. The case has deepened concerns Turkey, a key NATO ally bordering Iraq, Iran and Syria, is sliding towards greater authoritarianism under President Tayyip Erdogan.
Erdogan says a crackdown since last year's failed coup, which has seen more than 50,000 people jailed pending trial, is essential to maintain stability.
"This is ostensibly a trial of human rights defenders attending a workshop on an island in Istanbul, but in fact it is the Turkish justice system and Turkish authorities that are on trial," John Dalhuisen, Amnesty's director for Europe and Central Asia, said outside the courthouse.
Among those on trial are Amnesty International's Turkey director, Idil Eser, German citizen Peter Frank Steudtner and Swedish citizen Ali Gharavi.
Eight of the defendants were held in jail ahead of the trial.
The case has worsened Turkey's already fraught relations with the European Union it aspires to join.
German Chancellor Angela Merkel has said Turkey's 12-year-old attempt to join the EU should be halted, although Ankara said it remains determined to press on with its accession process.
