A Turkish court has acquitted 21 suspects in the trial of more than 270 people accused of plotting to topple the country's moderate Islamist government.

The same court sentenced former colonel Arif Doğan to 25 years and former police chief Adil Sacan to 14 years in jail for establishing a clandestine militia to kill thousands of dissenting Kurds.

The case, which began in 2007 with the discovery of 27 hand grenades in a house in Istanbul, has seen some of the country's most prominent figures detained and arrested.

The defendants face dozens of charges; from membership of Ergenekon, an underground terrorist organisation, to illegal possession of weapons. Among the other defendants are retired armed forces commander İlker Basbug and other military officers, politicians, academics and journalists.

Inside the courthouse, Mustafa Balbay, one of the suspects, said: "This trial is purely political. Today it's the government which is convicted, not us."

The ultra-nationalist network allegedly pursued extra-judicial killings and bombings in order to trigger a military coup and overthrow the government of Recep Tayyip Erdogan.

Security forces set up barricades around the courthouse in Istanbul after the defendants' supporters pledged to hold a demonstration against the trial. Critics of the case say there is little evidence for the charges and accused the government of trying to silence its opponents. The case is being seen as a key test in prime minister Recep Tayyip Erdogan's showdown with secularist and military opponents.