Prosecutors say the plotters operate inside the "deep state"
Fifty-six people, including two retired generals, journalists and academics, have gone on trial in Turkey accused of plotting to overthrow the government.
Prosecutors say they were members of a shadowy ultranationalist network - dubbed Ergenekon - which allegedly aimed to provoke a military coup.
The two generals, who are in their 60s, could face life in prison if convicted.
This is the second court case related to the Ergenekon case. Another 86 suspects went on trial in October.
The investigation has strained relations between the governing AK Party, which has its roots in political Islam, and the military, which considers itself the guardian of Turkey's secular constitution.
Last week, President Abdullah Gul approved a new law giving civilian courts the power to try military personnel suspected of threatening national security or having links to organised crime.
Forty-four of the defendants were present inside the courtroom at the heavily-guarded Silivri prison on the outskirts of Istanbul on Monday to hear the charges against them read out.
Sener Eruygur was not present in court on Monday because of ill-health
Gen Hursit Tolon, a former army commander, looked relaxed as he answered questions from the four-judge panel after being accused of masterminding a terrorist group and inciting armed rebellion against the government.
His co-accused, Gen Sener Eruygur, a former commander of the paramilitary gendarmerie forces, was not present because of ill-health.
According to the 1,909-page indictment, the two men "began implementing the coup plans they drew up in 2003-2004 while in office and continued their activities after they retired".
The allegations first surfaced in March 2007, when a magazine published excerpts from the purported diary of a former navy commander, which described how Gen Eruygur and several other senior officers had plotted coups but failed to secure the support of the heads of the armed forces.
After retiring, the indictment says, the two men used civil society groups to incite public opinion against the AKP-led government.
At the same time, it alleges, they helped set up Ergenekon, which is accused of being behind several violent attacks, including the bombing of a secularist newspaper in 2006 and an attack on the country's top administrative court in the same year, in which a judge died.
Targeting those key parts of the secular establishment were supposed to foment chaos and to provoke Turkey's military into launching a coup in defence of secular interests, it is alleged.
Other prominent suspected Ergenekon members who went on trial on Monday include two journalists who have frequently criticised the government, Mustafa Balbay and Tuncay Ozkan; two university rectors; and the head of the Ankara chamber of commerce.
The government denies the probe is designed to undermine its opponents
All the defendants deny the charges, saying they are politically motivated and designed to undermine the AK Party's opponents.
About 200 people demonstrated against the trial outside the court building on Monday, many holding portraits of Ataturk, the secularist founder of modern Turkey.
"This trial is a lie. They are fabricating evidence to arrest Ataturk's followers," one protestor, Suzan Demirten, told the Associated Press.
The BBC's David O'Byrne in Istanbul says it is unclear if the presiding judge will now decide to merge the proceedings with the ongoing trial of the 86 other suspects in the Ergenekon case, who include several other senior military personnel.
What is certain, however, is that few Turks doubt that at least some truth lies behind the accusations of coup plotting by elements of the military, our correspondent says.
And equally few doubt that whatever the result of the trials, the delicate balance of power between Turkey's political and military elites has changed irrevocably, he adds.