Three opposition MPs have received prison terms of between 12 and 35 years after being found guilty in the so-called Ergenekon conspiracy to overthrow the government of Prime Minister Tayyip Erdogan.
In Monday's proceedings so far, 21 people have been acquitted.
The case stems from a longstanding conflict between Erdogan and Turkey's secularist opponents, and involves some of the country's most senior miltary officers, business leaders, journalists and academics.
The defendants are accused of plotting high-profile attacks that prosecutors say were aimed at sowing chaos in Turkey to prepare the way for a military coup.
Ever since Erdogan's ruling Islamist-rooted Justice and Development Party (AK Party) came to power in 2002, tensions have been high with the army, which sees itself as a guardian of the secular state.
The prosecutions have helped Erdogan's government reshape Turkey's military and assert civilian control in a country that had seen three military coups since 1960.
Charges evidence was fabricated
Initially, the trial enjoyed broad popular support. It was seen as a way to end the military's meddling in politics. But concerns over the five-year-long case have steadily grown as the number of defendants increased — all of who are well-known critics of the government.
There are also claims that much of the evidence was fabricated and that defendants were denied basic legal rights.
The government denies such charges and claims the case ushers in a new era of democracy. But critics point out that the case symbolizes the transfer of one authoritarian rule for another.
Security forces have sealed off all roads leading to the courthouse in Silivri prison, just outside Istanbul, and the city's governor has warned that he will not allowed any demonstrations.