Turkish philanthropist Osman Kavala was sentenced on Monday to life in prison without parole.
Kavala was convicted of trying to overthrow the Turkish government by financing protests.
The 64-year-old had been in jail for four-and-a-half years without a conviction and denied the charges he faced over the Gezi protests, which began as small demonstrations in an Istanbul park in 2013 and snowballed into nationwide anti-government unrest.
The court also sentenced seven others to 18 years in jail each for aiding an attempt to overthrow the government and ordered their arrests.
The court said it decided to acquit Kavala of espionage charges due to lack of evidence.
The courtroom was packed with more than 200 people, including opposition members, Western diplomats and rights activists, Reuters reported.
Supporters of the accused yelled at the judges as the ruling was read. Many of them cried as the seven defendants, including architect Mucella Yapici, 71, were taken into custody.
"This is just the beginning; the struggle continues," the crowd chanted.
In his last words before the verdict, Kavala said the prosecutor's request for a life sentence was based on "evidence that is not evidence" and amounted to "an act of assassination by use of the judiciary".
Kavala played a major role in developing Turkish civil society before he was detained in 2017, from establishing a publishing house that aimed to foster social change after Turkey's 1980 coup to boosting culture through his Anadolu Kultur organisation.
Nils Muiznieks, Amnesty International's Europe director, said: "Today, we have witnessed a travesty of justice of spectacular proportions. This verdict deals a devastating blow not only to Osman Kavala, his co-defendants and their families, but to everyone who believes in justice and human rights activism in Turkey and beyond."
The European Court of Human Rights had called for Kavala's release in late 2019 and ruled his detention was meant to silence the philanthropist, whose civil society projects aimed to foster social change.
Emma Sinclair-Webb, Turkey representative for Human Rights Watch, said the verdict was an "active defiance against the Council of Europe".
Embassies of Turkey's western allies, including the US and Germany, echoed the call for Kavala's release last year, prompting threats by Mr Erdogan to expel their ambassadors.
Mr Erdogan has compared the Gezi protesters to Kurdish militants and those accused of orchestrating a failed 2016 coup.
He has accused Kavala of trying to overthrow the government, saying western allies would not release "bandits, murderers and terrorists" in their countries.
Kavala was acquitted in 2020 of charges related to the Gezi protests. Hours later, another court ordered his arrest on a charge of attempting to overthrow the constitutional order related to the coup attempt.
That court later ruled to release him on that charge but ordered his detention on an espionage charge in the same case.
Kavala's acquittal along with eight others in the Gezi trial was overturned last year and the case was combined with the other charges against him.