Critics saw the charges and resulting trial as a momentous bid by those in power to crack down on opposition voices and criminalize mass anti-government protests. A total of 16 people were accused of organizing or aiding in the 2013 protests in an attempt to violently overthrow the government.
A panel of judges ruled that civil society defender and ex-businessman Osman Kavala, the sole defendant still in jail, should be freed. It ordered seven defendants who live abroad to be brought in for questioning.
Supporters broke into applause and tears when a judge quickly read the panel's verdict for acquitting the activists instead of convicting them and imposing harsh sentences, as many had feared.
"It's a great verdict. Acquittal was the only thing that could be just. This is the verdict we should have been given two years ago," Andrew Gardner of Amnesty International told The Associated Press. "We will have to see what comes next but today is a decision to celebrate."
The protests at the center of the case started to oppose the planned development of a small park in central Istanbul into an Ottoman-style shopping mall. The demonstrations grew into a wider protest movement across Turkey, challenging Turkey's prime minister at the time and now president, Recep Tayyip Erdogan.
More than 300 people came to watch the trial Tuesday, joining lawmakers, foreign delegates and rights group members at a courthouse near the Silivri maximum security prison campus, on the outskirts of Istanbul. Hundreds of others waited outside.
Many observers burst into applause when Kavala entered the courtroom and after some of the testimony, prompting warnings from the court. The atmosphere was tense. The court rejected requests to hear the testimony of defense witnesses and to give the defendants more time to respond to the prosecutor's sentencing statement.
The tension built to an uproar when security forces tried to remove a defense lawyer from the courtroom. Officers in riot gear arrived after members of the audience and lawyers loudly voiced their objections.
"Complaints of the lawyer not being heeded by the court, statements by the defendants, really seemed to fall on deaf ears in terms of the panel of judges," Gardner said, describing the trial as a roller-coaster. "And then suddenly, for the judge to announce a not-guilty verdict for all, is incredible."
The prosecutor had sought a life sentence in solitary confinement without parole for Kavala, architect Mucella Yapici and Yigit Aksakoglu, who works on early childhood development and spent 221 days in pretrial detention. They denied trying to overthrow the government and say the protests were an exercise of democratic rights.
The prosecutor demanded 15 to 20 years in prison for six other defendants, among them filmmakers, a lawyer and an urban planner, for aiding an attempted overthrow.
Kavala, 63, founded a nonprofit organization, Anadolu Kultur, that focuses on cultural and artistic projects promoting peace and dialogue. He rejected the accusation that he organized and financed the 2013 protests. He said he took part in peaceful activities to defend the environment and the park, which is near his office.
Kavala was arrested in November 2017, four years after the protests. The European Court of Human Rights ruled for his immediate release in December, saying Kavala's extended time in custody served "the ulterior purpose of reducing him to silence" with a "chilling effect on civil society."
The Istanbul court did not heed the European court's order during two previous hearings but ruled in favor of Kavala's release Tuesday.
An estimated 3.6 million people joined the Gezi Park protests, according to government estimates, and thousands were injured. Police used tear gas and water cannons to disperse mostly peaceful protesters and have been criticized for excessive force.
The Turkish Bar Association puts the number of killed in the unrest at 15, including a police officer, but the prosecutor's indictment against the defendants says five were killed. The discrepancy stems from the inclusion of heart attacks and cerebral hemorrhages thought to be caused by pepper spray, as well as those killed in other protests during the same period.