Ilker Basbug, Turkey's former army chief, has gone on trial on charges of leading a terrorist group accused of plotting to overthrow the government of Recep Tayyip Erdogan, the prime minister.
Basbug raised a clenched fist and waved to supporters as the trial opened at the Silivri high security prison complex in Istanbul on Monday.
Basbug, chief of staff from 2008 to 2010, is accused of being a leader of a shadowy network dubbed "Ergenekon", behind a string of alleged plots against the Erdogan government.
His lawyer, however, said at the weekend, the case targeted not only Basbug but also "the Turkish armed forces and even, in political terms, the state".
"This is perhaps the longest and most seismic operation in Turkish judicial history," Al Jazeera's Anita McNaught, reporting from Istanbul, said.
"It dates back to 2007 when the Turkish government said they had uncovered evidence of a shadowy organisation called Ergenekon, which had been plotting for several years and in many ways and forms in collaboration with the Turkish military and judiciary to overthrow the democratically elected government of Turkey."
She said the resulting arrests, trials and detentions "have continued to climb up the ladder of seniority in Turkey until finally they reached the man who headed the Turkish armed forces between 2008 and 2010".
Basbug branded the case against him as tragi-comic when he was first detained in January. "He calls it psychological warfare," our correspondent said.
The trial encroaches on sensitive territory in a country that saw three coups in the second half of the 20th centrury.
The military has viewed Erdogan, a man with roots in political Islam, and his AK Party with deep suspicion since it
was first elected in 2002.
Since then the AKP has built up a huge majority in parliament, reformed the judiciary and used its authority, bolstered by economic success, to strip the military of the power it has enjoyed to make or break governments.
Dressed in a dark suit and tie, Basbug was the first of 29 defendants to confirm his identity to the bench of three judges on Monday. His answers to judges were to the point.
He waved to several co-defendants who called out to their former chief, and raised a clenched fist as a sign of solidarity with one old colleague also on trial.
The case against him features websites allegedly set up by the military to spread "black propaganda" against the
government until 2008.
Basbug is the most senior officer among hundreds of secularists facing conspiracy and terrorism charges.
"There are really two views in Turkey now, of what exactly this trial is about," Al Jazeera's McNaught said.
"There are people in Turkey who do believe - and there is as much evidence to sustain this - that there was an undercover group working in various forms ... but there are many Turks now who completely doubt the legitimacy of this whole process.
"So you have a country split about whether these trials are actually genuine or whether they are a witch-hunt for the political opponents of the Turkish government."