A probe into the shadowy Ergenekon network began more than two years ago, and hundreds of people have been detained in multi-stranded investigations, including several retired and some active members of the armed forces.
Seyfi Oktay, who served as justice minister in several coalition governments in the early 1990s, was detained after police searched two residences he owned in Ankara, according to the state-run Anatolia news agency.
"I have struggled for democracy all my life. This struggle will continue. My past is bright clean. I will respect the law," Oktay told reporters as he was escorted to a police car.
Many Ergenekon suspects have been held without trial, with detention orders renewed monthly.
With four civilian governments ousted by the military since 1960, the public initially welcomed the Ergenekon investigation. But critics have accused Prime Minister Tayyip Erdogan's AK Party of mounting a witch hunt against political opponents.
Aside for Oktay, police in Ankara, Antalya and Istanbul detained four lawyers and the vice chairman of the Labour Party, a small leftist party that has no seats in parliament.
