SILIVRI: A Turkish court on Monday sentenced a former army chief to life in prison in a high-profile trial of 275 people accused of plotting against the Islamic-rooted government, a ruling that sparked angry protests in the streets.

Police fired tear gas and water cannon at thousands of protesters outside the court near Istanbul after the verdicts were delivered in the divisive case, which resulted in lengthy prison sentences for the majority of the accused, including top brass, journalists and opposition lawmakers.

Ex-military chief Ilker Basbug, several other army officers as well as reporter Tuncay Vzkan and lawyer Kemal Keringsiz, were all sentenced to life in prison, while 21 people were acquitted.

The trial was seen as a key test in Prime Minister Recep Tayyip Erdogan's showdown with secularist and military opponents during his decade-long rule.

The defendants faced 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.

Journalist Mustafa Balbay, an elected deputy of the opposition Republican People's Party (CHP), was handed 34 years and eight months.

Another CHP deputy Mehmet Haberal, a professor, saw part of his 12.5-year sentence reduced and was allowed to walk free given time already served.

According to the private NTV television station, another 15 people were also convicted but immediately released.

Tensions were high outside the high-security tribunal in the town of Silivri, near Istanbul, throughout the hearing.

After the verdicts were announced, fierce clashes erupted between police and about 10,000 protesters on the Istanbul-Tekirdag highway, a few hundred metres (yards) from the court complex, an AFP photographer reported.

Protesters threw stones at riot police who responded with water cannon, tear gas and rubber bullets to break up the demo that was blocking traffic.

Istanbul governor Huseyin Avni Mutlu had on Friday said that demonstrations outside the court would not be allowed.

In Ankara, hundreds of people also took to streets in protest at the court ruling, chanting: “We are soldiers of Mustafa Kemal Ataturk,” a reference to modern Turkey's founder.

“This trial is purely political,” Balbay told an audience of MPs and journalists inside the heavily guarded court building ahead of the verdicts.

“Today it's the government which is convicted, not us.”Basbug, 70, led Turkey's military campaign against the rebel Kurdistan Workers' Party (PKK) for many years, only to find himself accused in his retirement of having led a terrorist group himself.

“I came to fight injustice, to defend our rights. I am an ordinary Turkish citizen, I have no ties with the suspects,” housewife Ebru Kurt said as she joined protesters in Silivri.

“I am not saying that all the people in jail are innocent, but I am convinced that most of them have spent years in jail even though they have done nothing wrong.”

The verdicts come after Turkey was rocked in June by mass protests that presented Erdogan's government, seen as increasingly authoritarian, with its biggest public challenge since it came to power.

The mass coup plot trial has polarised the country, with Turkey's secular opposition denouncing the lengthy proceedings, which began in 2008, as a witch hunt aimed at silencing government critics.

But 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.

And 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.

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 Erdogan's 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.