The 26 defendants join an estimated 5,500 people on trial in 95 separate prosecutions, including some on terrorism charges, according to Turkish rights groups, linked to the unrest that challenged Prime Minister Tayyip Erdogan's decade in power.
In the latest court case, five of the defendants are accused of forming a criminal organisation - which they deny - and could face up to 30 years in prison. The indictment cites, among its primary pieces of evidence, messages on social-media websites Twitter and Facebook that encouraged people to come to public protests.
Andrew Gardner, Turkey researcher for Amnesty International, said: "It is an absolutely scandalous prosecution that should have never been brought to court.
"The indictment is completely without evidence of a crime, in the understanding of international human rights laws and Turkey's own laws. The right to peaceful assembly is being put on trial."
Most of the defendants are part of Taksim Solidarity, an ad hoc collection of engineers, architects, doctors, business owners and activists who opposed Mr Erdogan's plans to redevelop Taksim Square, including razing Gezi Park to build a shopping centre, amid a construction boom that has transformed Istanbul.
The small environmental movement to save Gezi, one of the few public green spaces in Europe's biggest city, quickly mushroomed into nationwide anti-government protests in June 2013, with many accusing Mr Erdogan of authoritarianism.
At least six people died in clashes with police, and the Turkish Medical Association says 10,000 were seriously hurt.
Other defendants in the latest trial risk sentences of between one and three years for offences such as breaking the law on public assembly, according to the indictment.