Gen Sener Eruygur has previously organised anti-AKP demonstrations
Two retired generals have been remanded in custody in Turkey over a suspected plot to overthrow the government.
Sener Eruygur and Hursit Tolon were the highest-ranking officials among 21 people rounded up in police raids across Turkey on Tuesday.
They are the first military personnel to be arrested in connection with the investigation. They deny wrongdoing.
All those rounded up were suspected of being members of a shadowy ultra-nationalist group known as Ergenekon.
Both men are known critics of the government and Gen Eruygur helped to organize a series of anti-government rallies last year.
Gen Eruygur's lawyer told the state-run Anatolia news agency: "A commander who served his country for years was arrested today with false accusations. We accept none of these accusations."
Ozgur Meric Tuna, Gen Tolon's lawyer, said his client had not been judged fairly and that he would appeal to secure his release.
Eight people have now been formally arrested, including a prominent businessman.
Five others, including a senior journalist, were released on Friday but barred from leaving the country while the case continued.
A police investigation against Ergenekon was launched in June 2007 after explosives were found in a house in Istanbul.
Turkish media said that a secret plan was seized during Tuesday's raids, which called for the launch of illegal protests on 7 July across 40 provinces, assassinations and clashes with security forces.
The detentions came as Turkey's governing AK Party was defending itself in court against accusations that it is trying to impose Sharia law and should be banned.
The AKP has denied accusations that it is trying to intimidate and silence its opponents.