By Ece Toksabay

ISTANBUL (Reuters) - Hundreds of Turks protested on Tuesday after prosecutors demanded life sentences for more than 60 retired generals, politicians and others, accused of plotting to overthrow Prime Minister Tayyip Erdogan's government.

Prosecutors summed up their case on Monday in the so-called Ergenekon trial and demanded life sentences for the retired top armed forces commander Ilker Basbug and opposition lawmakers Mustafa Balbay and Mehmet Haberal, among others who are considered staunch secularists and nationalists.

Sentences of up to 15 years in prison were sought for another 96 defendants out of a total of 275, court documents showed, for being members of what prosecutors called the "Ergenekon terrorist organization". They were also charged with conspiring to overthrow Erdogan's government which has its roots in political Islam.

Protesters waving Turkish flags gathered at Istanbul's central Taksim Square, where there is a statue of Mustafa Kemal Ataturk, founder of the secular republic.

"We made our vows, Silivri prison will be torn down," the crowd chanted, referring to the jail where defendants have spent years in detention during the long-running trial. Many carried pictures of Ataturk.

Prosecutors have described Ergenekon as a shadowy network that carried out extra-judicial killings, bombings and other attacks. Supporters of the prosecution have said that locking up the suspects has made Turkey a more stable place and allowed Erdogan to tackle sensitive issues such as negotiating a settlement of the 28-year-long insurgency by Kurdish militants.

Erdogan says he is fighting to stamp out anti-democratic forces, but critics see the case as a ploy to stifle opposition, part of a grand plan by the leader to tame the secularist establishment, including an army that intervened to topple governments four times in the second half of the 20th century.

"They made up an organization to lock up Turkey's enlightened people. The real fight is against our country's independence," said Serap Sofuoglu, 50, a retired teacher.

The media has also been divided over the trial.

"The trial has been marred by procedural mistakes and controversy about statements of secret witnesses ... People having no connection to each other have been brought together to assert that they were members of an organization," said Cumhuriyet newspaper, which is close to the main opposition Republican People's Party.

But newspapers that generally back Erdogan's government applauded the prosecutors' case.

"The end of the Ergenekon trial will mark the end of an era, wrote Mehmet Baransu in Taraf, a liberal newspaper that has carried leaks from the investigation over the years. "Millions would be crying now if they had achieved their goals."

The defendants will have the opportunity to make their final defenses, a process which lawyers expect will take another few months, before a verdict is announced.

The investigation into the alleged conspiracy, which surfaced in 2007 when police discovered a cache of weapons in Istanbul, was initially welcomed by a public eager to see an end to the "Deep State," a network of militant secularists and ultra-nationalists long believed to have been pulling the strings of power.

But dissenting voices have grown in recent years, with the European Commission expressing concern about the handling of Ergenekon and other conspiracy trials.

In September, a court in Silivri jailed more than 300 military officers in another trial for plotting to overthrow Erdogan's government.

(Writing by Ece Toksabay; Editing by Ayla Jean Yackley and Jon Hemming)