* Court jails more than 300 officers
* Key step in ending army's political sway
* Critics say trial silenced opponents
ISTANBUL, Sept 23 (Reuters) - The jailing of hundreds of Turkish army officers including top generals accused of plotting to topple Prime Minister Tayyip Erdogan underscored how far he has come in gaining control of the country's once all-powerful military.
But Erdogan, 10 years in power, must grapple with suspicions among critics and even some sympathisers that he is using this and other coup investigations to silence opposition as he sets about taming a militant secularist establishment. Far from flinching, he may seek more power in a revamped presidency.
The verdict against 325 officers at the end of the 21-month trial on Friday would have been unthinkable a decade ago, when generals regularly intervened in policy-making as self-appointed guardians of Turkish secularism.
Judges in the case, dubbed Sledgehammer, handed down prison sentences ranging from six to 20 years against the officers for plotting to wreck Erdogan's rule almost 10 years ago, soon after his Islamist-rooted party swept to power with the biggest share of the vote in decades.
Hilmi Ozkok, who was head of the armed forces at the time, rejected accusations the court's decision was driven by revenge.
"The ruling will serve as a deterrent and has a lesson for everyone ... in understanding how much Turkey and the rest of the world has changed," Ozkok told Milliyet newspaper on Sunday.
Turks reading such words from the mouth of the former armed forces chief will gain a measure of the scale of change since Erdogan's AK party was first elected in 2002. The generals then made no secret of their disdain for a man who had served a brief prison sentence for relgious incitement and had backed a short-lived Islamist government they eased from power in 1997
When AK was elected for a second term in 2007 with an even larger margin of victory, an emboldened Erdogan launched a series of investigations into officers, lawyers, politicians, journalists and others that exposed several alleged conspiracies against the government.
The plots consisted of plans to foment unrest and pave the way for an army takeover. Sledgehammer, a war game scenario played out at a barracks in Istanbul in March 2003, included plans to bomb historic mosques in Istanbul and trigger conflict with Greece.
For many, it was all to easy to believe. Turkey's military, NATO's second biggest, staged three outright coups between 1960 and 1980 and pressured a fourth government, the first Islamist-led, from power in 1997.
ENDING 'MILITARY TUTELAGE'
Under Erdogan, a devout Muslim, curbs on religion have been relaxed. Women are allowed to more freely wear the Islamic headscarf, alcohol is heavily taxed, and students at religious high schools are able to more easily attend university.
Journalists complain of pressure to write favourable stories about the government, and a number of writers are among those arrested under another plot investigation, "Ergenekon".
"This (Sledgehammer) case is an important step towards ending the army's political role but it's not enough to stop it completely," said Sahin Alpay, professor of political science at Bahcesehir University and a columnist for Zaman, seen as close to the government.
"Now we need a new constitution and laws that place the army under civilian supervision and reform military schools to reflect the values of a liberal democracy," he said.
A new constitution is now under consideration to replace a restrictive code inherited from the military after a 1980 coup. Turkey may well emerge from the debate with a presidential republic and a powerful president in Erdogan.
Alpay acknowledged there were questions about the case with so many defendants on trial at once, the judges' refusal to allow in some defence evidence and the lengthy sentences.
A key issue at appeal is likely to be the defence's inability to submit legal expert testimony that computer documents submitted as evidence appeared fake.
Defence lawyers said they would appeal the verdict this week to Turkey's upper court and, if necessary, eventually apply to the European Court of Human Rights.
Generals Cetin Dogan and Halil Ibrahim Firtina and retired admiral Ozden Ornek, who were considered Sledgehammer's ringleaders were given life terms, reduced to 20 years because the coup plot had failed.
Critics of the government have said the trial was a purge of the government's opponents in the army's ranks.
"This isn't really a legal case," said Pinar Dogan, Cetin Dogan's daughter and a lecturer at Harvard University. "It's a wider operation with powerful forces behind it ... and we see those with firm secularist beliefs are the ones targeted."
She said her mother Nilgul Dogan, 60, faces three years in prison in a separate case for planting a rose bush near the courthouse to protest against the trial.
Others said the case failed to go far enough.
Gultan Kisanak, co-chairwoman of parliament's pro-Kurdish Peace and Democracy Party, said the indictment did not include crimes she says were committed in a 28-year long war with the Kurdistan Workers Party (PKK) that has killed more than 40,000 people.
"Military tutelage, a tradition of coups, contra-guerrilla activities, extrajudicial killings and other dark events are all part of our recent political history," she said.
"While we need a process to confront and reconcile our past, the government instead opted to settle its own scores."
Sledgehammer is one of a series of trials that has raised questions about whether the government is using the courts to silence political opponents.
Others include the "Ergenekon" case, which involves a web of alleged plots against Turkey's government, and the "KCK" trials which accuse thousands of Kurdish journalists, academics, lawyers and others of belonging to the PKK, viewed as a terrorist group by the United States and Europe for its campaign of violence for greater autonomy in southeastern Turkey.
The court ruling also has the potential to undermine morale in the military as it battles the PKK in the heaviest fighting in more than a decade and faces a growing challenge maintaining security along its southern border with war-torn Syria.
A hundred or so people gathered near Taksim Square in central Istanbul to protest the verdict.
"This case was an effort to silence those who defend the secular republic," said Hanife Kopuz, 55, clutching a cloth banner of Mustafa Kemal Ataturk, who founded the modern Turkish republic from the ruins of the Ottoman Empire.
"This may be a turning point, reducing support for the government. They can't stay in power forever but I fear what they will leave in their wake," she said.