A Turkish court will on Monday deliver its verdict in the trial of 275 people including a former army chief accused of plotting to overthrow the country's Islamic-rooted government.

Among the defendants in the massive case -- seen as a key test in Prime Minister Recep Tayyip Erdogan's showdown with secularist and military opponents -- are ex-military chief Ilker Basbug and other army officers as well as lawyers, academics and journalists.

They face dozens of charges, ranging from membership of an underground "terrorist organisation" dubbed Ergenekon to arson, illegal weapons possession, and instigating an armed uprising against Erdogan and his Justice and Development Party (AKP), which came to power in 2002.

Basbug, who led the army between 2008 and 2010, rejects all the charges against him. Turkey's secular opposition has denounced the lengthy trial, which began in 2008, as a witch hunt aimed at silencing government critics.

The 2,455-page indictment accuses members of Ergenekon -- an alleged shadowy network of ultranationalists trying to seize control in Turkey -- of a string of attacks and political violence over several decades to stir up unrest.

They include a shooting at Turkey's top administrative court in 2006 which killed a judge and which the state prosecutor believes was instigated by a retired general, and a grenade attack against the opposition Cumhuriyet newspaper's Istanbul headquarters blamed on the army command that same year.

Pro-government circles have praised the Ergenekon trial as a step towards democracy in Turkey, where the army violently overthrew three governments in 1960, 1971 and 1980.

In 1997, the army pressured then Islamic-leaning prime minister Necmettin Erbakan, the political mentor of the current premier, into stepping down in what was popularly dubbed a "post-modern coup" strategy.

However, some human rights advocates and defenders of the political legacy of the founder of modern secular Turkey, Mustafa Kamal Ataturk, see the trial as an attempt to weaken the secular opposition. They have questioned the strength of the evidence against the defendants, including the use of anonymous witnesses.

In his defence before the court in June, Basbug lashed out at the "injustice" deliberately made to weaken the Turkish army which he said must be prepared to take all measures to protect the nation.

"There's a big injustice today deliberately made against the Turkish army," Basbug said.

"The Turkish army is a big institution. That's to say, it is an open target. Its activities are publicised. It would not be difficult to find incidents open to manipulation or abuse within a big institution."

But Basbug emphasised that "the reason for this army's existence is only to serve the Turkish nation."

Thousands of supporters of the defendants were expected to turn out on Monday at the court, where Turkish authorities have stepped up security, doubling the number of metal barriers in case of unrest.

On Friday, Istanbul governor Huseyin Avni Mutlu said a planned demonstration in front of the court house would not be allowed and would be deemed "illegal."

"Do not come ... in vain," he told potential protesters.

Mutlu said that only lawmakers, lawyers, suspects and members of the press would be allowed to follow the hearing inside the courtroom, while planned demos outside would be banned.

On Wednesday, around 20 retired officers demonstrated with their families in front of the army chief's headquarters in Ankara to denounce what they called political trials.

Prosecutors claim that Ergenekon, named after a mythical place in central Asia believed to be the homeland of Turks, is made up of loosely connected branches with an eventual goal of toppling the AKP government and restructuring Turkey on a nationalist footing.

The network was uncovered in June 2007 when weapons and explosives were discovered during an anti-terrorist operation in an Istanbul suburb.

The trial is one of a series of cases in which members of the Turkish army, the second biggest in NATO, have faced prosecution for alleged coup plots against an elected government.

In a separate case in September dubbed "Sledgehammer", more than 300 hundred active and retired army officers, including three former generals, received prison sentences of up to 20 years over a 2003 military exercise alleged to have been an undercover coup plot.