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 verdict that sparked angry protests in the streets.

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

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.

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

"The final say belongs to the people," local media quoted Basbug as saying.

"Those who have always stood by ... justice have a clear conscience. I am one of those people."

Fierce clashes erupted between police and about 10,000 protesters near the court complex after the verdicts were announced, an AFP photographer reported.

Protesters threw stones at riot police who responded with water cannon, tear gas and rubber bullets to break up the demonstration, which blocked traffic on the Istanbul-Tekirdag highway near the court.

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.

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

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 and opposition MP Mustafa Balbay was handed 34 years and eight months, while another opposition deputy Mehmet Haberal had his sentence reduced and walked free given time already served.

Another 15 people were also convicted but immediately released, the private NTV television station said.

"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."

The heavy sentences handed down to journalists sparked criticism from the Organisation for Security and Cooperation in Europe (OSCE), accusing European Union hopeful Turkey of violating free speech.

"I am deeply alarmed by today's convictions and harsh sentences that are of unprecedented length and severity in the entire OSCE region," OSCE media freedom representative Dunja Mijatovic said.

"Criminal prosecution of those with dissenting views violates the fundamental human right to free expression and the country's OSCE commitments to develop and protect free media," Mijatovic added.

Basbug, 70, led Turkey's military campaign against the rebel Kurdistan Workers' Party (PKK) for many years, only to be accused in retirement of having led a terrorist group himself.

In a Twitter post on Sunday, the former general described the trial as "a black stain on the glorious history of the Turkish state and its army".

The verdicts, which were expected to be appealed, come after Turkey was rocked by mass protests in June 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.

Prosecutors said Ergenekon, named after a mythical place in central Asia believed to be the homeland of Turks, was made up of loosely connected branches with an eventual goal of toppling 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.

Deputy Prime Minister Bulent Arinc declined to comment on the verdicts on behalf of the government, but personally urged respect for the independent judiciary.