An Istanbul prosecutor requested on Monday life sentences for 24 suspects, including former army chief Ilker Basbug, journalists and other retired generals, accused of attempting to overthrow an elected Turkish government.

The prosecutor submitted his 2,271-page final opinion on 275 suspects involved in the four-year long probe dubbed the Ergenekon case, named after the shadowy criminal gang which he argued was a "terrorist organisation."

He sought aggravated life sentences for the 24 top suspects accused of "attempting to overthrow the Turkish government" and "trying to prevent it from functioning entirely or partially."

Dozens of others are facing seven-and-a-half to 15 years in prison for involvement in the ultra-nationalist network, accused of being behind several plots to overthrow the Islamic-rooted government of Prime Minister Recep Tayyip Erdogan.

The prosecutor's opinion is expected to be followed by a final verdict, after months of delays due to technical disruptions in the trial that spanned almost 300 hearings since 2008.

Pro-government groups have praised the trial as a step toward democracy that will end a tradition of political interference in Turkey, where the once omnipotent army incapacitated four governments in 50 years.

But critics say the case is based on shaky evidence to take "revenge" on circles opposing Erdogan's Islamic-leaning government, in power since 2002.

The indictments accuses suspects of a string of attacks over several decades, including a shooting at a top Turkish administrative court in 2006 which killed a judge and a grenade attack against opposition Cumhuriyet daily's Istanbul headquarters.

Both attacks were initially blamed on Islamists, but the state prosecutor believes they were instigated by the army command at the time to foment chaos to stain the government and restructure Turkey on a nationalist framework.

In a separate case, more than 300 active and retired army officers, including three former generals, received prison sentences of up to 20 years in September after the same court ruled that a military exercise dubbed "Sledgehammer" in 2003 was an undercover coup plot.