Prosecutors say the detainees either belonged to or were linked to Ergenekon, a nationalist-secularist group that planned to assassinate politicians and prompt a military coup to oust the Justice and Development party (AK).  

Lieutenant sought

Unal Inanc, a journalist specialising in judicial affairs, and Erhan Goksel, a political analyst and the owner of a leading polling company, were also detained.

Police are searching the Ankara office of the private ART television station, an opponent of the AK, which has its roots in political Islam, and of the Turk Metal union headquarters.

Live footage showed police leading Ozbek out of his Ankara home amid protests from dozens of supporters outside.

The state-run Anatolian news agency said that a search was also under way for an army lieutenant in western Turkey. A regional police chief and an officer have been taken to Istanbul for questioning.

Hand grenades

The investigation into Ergenekon began when hand grenades were discovered in a property in Istanbul in June 2007.

Soldiers check barriers outside the Ergenekon trial in October [File: AFP]

In October last year, 86 people, including retired army officers, politicians, journalists and underworld figures went on trial, accused of belonging to a "terrorist organisation" and of plotting to overthrow the government.

The trial is ongoing and has caused much debate in Turkey. 

The investigation initially received support for countering the so-called "deep state," a term used to describe security forces acting outside the law, often in collusion with the criminal underworld, to protect what they see as Turkey's interests.

But many now charge that it has become a tool to "bully" government opponents following a failed attempt last year to have the AK banned for anti-secular activities.