A total of 325 military officers were convicted in September in a historic case that has helped curtail the military's hold on politics, and received prison sentences ranging between six and 20 years. Thirty-six others were acquitted.
Lawyers on Monday began presenting their arguments in the appeals case that is expected to last for days.
The trial has been hailed as a break with a tradition of military impunity and a move toward greater democracy. But the case has also been marred by some flaws, including the alleged fabricating of evidence. Lawyers complained throughout the initial trial that they were barred from presenting key witnesses or evidence.
Critics have denounced trial as a ploy to intimidate secular opponents.
The case "is the biggest conspiracy in the history of the Turkish Republic," the state-run Anadolu Agency quoted Kazim Yigit Akalin, the first of the lawyers to present arguments, as telling the appeals court. "While even at wartime, such a high number of officers cannot be rendered ineffective, this has been achieved in Turkey through a conspiracy."
Turkish generals, who have long regarded themselves as protectors of Turkey's secular traditions, have staged three coups between 1960 and 1980 and forced an Islamist government to quit in 1997.
Last week, Turkey's parliament amended an armed forces regulation which the generals have long relied on as legal grounds for intervening in politics. The regulation, which states the military's duties as protecting the Turkish republic was amended to define its role as protecting the Turkish homeland from foreign threats.