Retired military chief of staff General Ilker Basbug was sentenced to life for his role in the "Ergenekon" conspiracy to overthrow the government of Prime Minister Tayyip Erdogan.
The five-year trial has been one of the turning points in a decade-long battle between Islamist-rooted Prime Minister Tayyip Erdogan and a secularist establishment that ran Turkey since its birth as a modern state under founder Mustafa Kemal Ataturk.
Announcing verdicts on the 275 defendants in the case, the judges also sentenced three serving parliamentarians from the opposition Republican People's Party (CHP) to between 12 and 35 years in prison.
Prosecutors say a network of secular nationalists, code-named Ergenekon, pursued extra-judicial killings and bombings in order to trigger a military coup, an example of the anti-democratic forces which Erdogan says his AK Party has fought to stamp out.
The judges also passed life sentences on a former commander of Turkey's prestigious First Army, a retired gendarmerie commander, the leader of the leftist Workers' Party Dogu Perincek and high-profile journalist Tuncay Ozkan.
Critics, including the main opposition party, have said the charges were trumped up, aimed at stifling opposition. They say the judiciary was subjected to political influence in the case.
Six judges took it in turns to read the verdicts, sentencing defendants for membership of the "Ergenekon terrorist organisation." Booing by defence lawyers, opposition politicians and some journalists in court turned to applause as half of the defence lawyers stormed out in protest at the sentences.
"We are Mustafa Kemal's soldiers," the defendants and defence lawyers chanted in reference to Ataturk. "Damn the AKP."
Earlier, security forces fired tear gas in fields around the courthouse in the Silivri jail complex, west of Istanbul, as defendants' supporters tried to protest against the trial.
Erdogan has dominated Turkish politics for a decade, winning three elections in a row and presiding over an economic boom. However, his opponents accuse him of having little tolerance for criticism, an accusation that helped fuel street protests in June which began as a demonstration against development plans for an Istanbul part but quickly swept across the country.
Monday's verdicts could lead to further demonstrations.
THEATRE
On Monday, main access roads were shut and protesters' buses prevented from reaching the area. Hundreds of the defendants' supporters attempted to cross the fields to reach the court, but police with riot shields blocked their advance.
"The day will come when the AKP will pay the price," some chanted on the approach road to the court, where hundreds of riot police and paramilitary gendarmes were positioned.
"This is Erdogan's trial, it is his theatre," Umut Oran, an opposition parliamentarian with the CHP party, told Reuters.
"In the 21st century for a country that wants to become a full member of the European Union, this obvious political trial has no legal basis," he said at the courthouse.
Deputy Prime Minister Bulent Arinc said the case was an "important trial for Turkey" but the government could not comment on the outcome.
"This is a judicial decision. We all have to adhere to it whether we like it or not. We are not people who personally rejoice or applaud when someone is convicted or detained," he said.
The threat of a coup is not far-fetched: the secularist military staged three coups in Turkey between 1960 and 1980 and pushed the first Islamist-led government out of office in 1997.
But Erdogan has chipped away at the army's influence since his AK Party came to power, including in court with the Ergenekon case - named for a mythical valley from Turkish lore - and a separate trial over a plot known as "Sledgehammer".
Erdogan has denied interfering in the legal process, stressing the judiciary's independence. But he has criticised the prosecutors handling the case and expressed disquiet at the length of time defendants have been held in custody.
Among the defendants were military officers, politicians, academics and journalists. They denied the charges. Twenty-one of the defendants were acquitted as the court announced verdicts one by one.
"He is here because he is a patriot, he committed no crime and this court failed to prove he did," said Tuncay Ozkan's daughter Nazlican, a 20-year old student who has been attending her father's trial since she was 15.
"It is impossible for me to recognise this ruling."
Basbug criticised the court on his Twitter account after the verdicts were announced.
"If society questions the independence of judges in a country, if it harbours doubts about whether its judgements are lawful, you cannot claim there is supremacy of law in that country," he said. "Those on the side of the truth and righteousness - that is, on the side of justice - have a clear conscience. That is how I am."
Last September, the court, in the Silivri suburb outside Istanbul, jailed more than 300 military officers for plotting to overthrow Erdogan a decade ago in the "Sledgehammer" plot.
The government's control over NATO's second largest army was illustrated on Saturday when Ankara appointed new military commanders in an overhaul of the top ranks, forcing the retirement of a senior general regarded as a government critic.
The Turkish public initially welcomed the Ergenekon trial on the grounds it would bring to account the country's "Deep State" - an undefined network of secularists long believed to have been pulling the strings of power in Turkey.
As the court proceedings advanced, criticism grew, however. The European Commission, which is negotiating Turkey's bid to join the EU, also expressed concern.
"We were all happy when this court case started because we thought it was an effort to clean up the Deep State. But we soon realised it was an effort to clean up political opponents," said Nedim Sener, an investigative journalist accused of links to Ergenekon and still on trial in a related case.
