* Basbug held in prison with other alleged conspirators
* Ultra-nationalists accused of trying to topple government
* Pursuit of top generals a watershed for Turkey -- analyst
* Trial seen as part of struggle between AK and secularists (Adds Basbug quotes, fresh analyst comments, details)
By Daren Butler and Can Sezer
ISTANBUL, Jan 6 (Reuters) - Former Turkish armed forces chief General Ilker Basbug spent his first night behind bars on Friday, charged with trying to overthrow the government in an unprecedented development likely to exacerbate tensions with the military.
Basbug, who retired in 2010, is the highest-ranking officer to be caught up in the so-called Ergenekon case, a long-running crackdown on EU candidate Turkey's once all-powerful military and secularist establishment.
The former general was taken from an Istanbul courthouse in the early hours of Friday for a health check before being transported in a police convoy to Silivri prison, some 80 km (50 miles) west of the city, where hundreds of defendants in the Ergenekon case are being tried in a specially-built courtroom.
"The Republic of Turkey's 26th general chief of staff has been remanded in custody for forming and directing a terrorist group. I leave it to the great Turkish nation to judge," Basbug said as he was lead from the courtroom.
The decision to send Basbug to jail came hours after prominent Turkish journalists on trial over alleged ties to the ultra-nationalist Ergenekon network said the charges against them were "a massacre of justice".
Ergenekon is accused by prosecutors of being behind multiple conspiracies against Prime Minister Tayyip Erdogan's AK Party government, and several hundred suspects, including retired senior military officers, academics, lawyers and journalists, have been detained in cases related to the network.
Investigations into Ergenekon have spiralled since they first opened in 2007, and critics accuse Erdogan's government of scaremongering to silence opponents. The government denies any such motives.
Basbug, facing charges of "gang leadership" and seeking to unseat the government by force, told the court after seven hours of questioning by prosecutors that he rejected the charges and described them as "tragicomic", broadcaster NTV reported.
"To hear such an allegation hurts my pride as a general who has done his duty to the country and state with honour. Accusing a chief of general staff of forming a terrorist group is the biggest punishment I could be given," he was quoted as saying.
Basbug's lawyer said he would challenge the decision to jail him pending trial, state-run Anatolian news agency reported.
"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 military affairs analyst Lale Kemal.
"I would not be surprised if we see some commanders resign (if Basbug is remanded in custody) but I do not expect this to bring serious instability to Turkey," she said.
Turkish media reports this week suggested senior commanders could resign if Basbug was charged in the case. The General Staff subsequently issued a statement denying those reports but speculation about possible resignations continued.
"There is every possibility there will be resignations if cases continue to be brought like this," said security analyst Gareth Jenkins. "Morale is already at rock bottom. It is already affecting operational capability," he said.
Last July, Basbug's successor and the heads of the army, navy and air force resigned in protest at the detention of more than 200 officers charged in a separate alleged conspiracy against the government, dubbed "Operation Sledgehammer".
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 EU accession.
The current investigation of Basbug centres on allegations that the military set up websites to spread anti-government propaganda to destabilise Turkey.
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 AK party, which has roots in a banned Islamist party and swept to power in 2002, and a secularist establishment including officers, lawyers, journalists and politicians.
Turkey is currently holding nearly 100 members of the media in jail, one of the highest numbers worldwide, in a crackdown that critics and rights groups say blights Muslim Turkey's image as a role model for democracy in the Middle East.
Late on Thursday another court in Istanbul rejected a request from prominent journalists to be released after nine months in custody on trial for links to the Ergenekon network.
"I am here because I am a journalist looking for the truth," said Ahmet Sik, who has written books about the infiltration of the police by an Islamist movement led by Fethullah Gulen, a Muslim theologian based in the United States and considered close to parts of the ruling AK Party.
The EU and the United States have raised their concerns over the arrests of journalists in Turkey. But with the economy growing rapidly and Turks tasting unprecedented prosperity and political stability, public outcry has so far been muted.
In Washington, the State Department said it was monitoring the trial and noted that U.S. Secretary of State Hillary Clinton had repeatedly urged Turkish authorities to address concerns about freedom of expression and freedom of the media.
"I don't think the secretary left the Turkish government in any doubt about where we stand on the press freedom issue," State Department spokeswoman Victoria Nuland told a briefing.
Also on trial is Nedim Sener, an award-winning journalist who has written about police negligence in failing to prevent the 2007 murder of prominent Turkish-Armenian journalist Hrant Dink.
Government officials say the journalists are on trial for criminal activities, not because of what they wrote. (Additional reporting by Simon Cameron-Moore; Writing by Daren Butler and Ibon Villelabeitia; Editing by Rosalind Russell)