Among the defendants in the high-profile case - seen as a key test in Prime Minister Recep Tayyip Erdogan's showdown with secularist and military opponents - are ex-military chief Ilker Basbug and other army officers as well as lawyers, academics and journalists.
They face dozens of charges, ranging from membership of an underground "terrorist organisation" dubbed Ergenekon to arson, illegal weapons possession, and instigating an armed uprising against Erdogan and his Justice and Development Party (AKP), which came to power in 2002.
Turkey's secular opposition has denounced the lengthy trial, which began in 2008, as a witch hunt aimed at silencing government critics.
Pro-government circles have praised the Ergenekon trial as a step towards democracy in Turkey, where the army violently overthrew three governments in 1960, 1971 and 1980.
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In 1997, the army pressured then Islamic-leaning prime minister Necmettin Erbakan, the political mentor of the current premier, into stepping down in what was popularly dubbed a "post-modern coup" strategy.
On Friday, Istanbul governor Huseyin Avni Mutlu said a planned demonstration in front of the court house would not be allowed and would be deemed "illegal."
"Do not come ... In vain," he told potential protesters.
Mutlu said that only lawmakers, lawyers, suspects and members of the press would be allowed to follow the hearing inside the courtroom.
Prosecutors claim that Ergenekon, named after a mythical place in central Asia believed to be the homeland of Turks, is made up of loosely connected branches with an eventual goal of toppling the AKP government and restructuring Turkey on a nationalist footing. The network was uncovered in June 2007.