A prosecutor called for a former head of Turkey's powerful armed forces to be remanded in custody on Thursday pending trial for allegedly attempting to overthrow the government, a move likely to exacerbate long-running tensions with the military.

General Ilker Basbug, who retired in 2010, is the highest-ranking officer to be caught up in a widening probe into the so-called Ergenekon network, an ultra-nationalist group accused by prosecutors of conspiring to topple the government.

The prosecutor asked for Basbug to be remanded in custody on charges of gang leadership and seeking to overthrow Prime Minister Tayyip Erdogan's government, the state-run Anatolian news agency said after Basbug was questioned for seven hours.

A court was expected to decide late on Thursday or overnight whether to accept the state prosecutor's request, Anatolian said. Such a move would have been unthinkable a few years ago when the armed forces held sway.

The fact that prosecutors are now touching senior generals is a turning point in the democratisation process of Turkey. Many were sceptical that prosecutors would go this far, said Lale Kemal, a military affairs analyst for the Zaman and Taraf newspapers.

I would not be surprised if we see some commanders resign (if Basbug remanded in custody) but I do not expect this to bring serious instability to Turkey, she said. The military realises it can no longer resist the democratisation process in Turkey and deceive the public.

Earlier Basbug, the first former chief of the armed forces to testify as a suspect in a criminal case in a civilian court, arrived at the Istanbul courthouse, looking relaxed in a dark suit, to answer questions from prosecutors in a closed session.

FALL OF THE 'PASHAS'

Several hundred defendants, including retired senior officers, lawyers, academics and journalists, have been put on trial in cases relating to the investigation.

Nicknamed pashas, a title dating back to Ottoman times, Turkey's once untouchable generals have seen their influence decline as Ankara pushes reforms aimed at strengthening civilian rule and winning Turkey's accession to the European Union.

The current investigation centres on allegations that Turkey's military set up websites to spread anti-government propaganda to destabilise Turkey.

Murat Yetkin, editor of the Hurriyet Daily News, said Turks were deeply curious about recent events.

The country has to live through this thing. People will really want to know what has happened in the near past...and what has happened today, he said.

Turkey's military, NATO's second-largest army, has long seen itself as the guarantor of the country's secular constitution, and had staged three coups between 1960 and 1980 and pressured another government from power in 1997.

The Ergenekon case is seen as part of a power struggle between Erdogan's ruling AK party, which has roots in a banned Islamist party and swept to power in 2002, and an old secularist establishment including military officers, lawyers, journalists and politicians.

Chief of general staff from 2008 to 2010, Basbug has in the past said reports of military plots to undermine the government were part of a smear campaign to divide the armed forces and pledged he would never tolerate coup activities.

The Internet Memorandum case is just one of many strands of investigations into Ergenekon that began five years ago.

Opposition parties have accused the government of using Ergenekon to go after AK's critics. The government denies this.

Retired General Hilmi Ozkok, also a former chief of the general staff, testified in the Ergenekon case in 2009 but only as a witness, not a suspect.

(Additional reporting by Simon Cameron-Moore; Writing by Ibon Villelabeitia and Daren Butler; Edited by Ben Harding and Jon Boyle)