The Ankara chief prosecutor's office began the probe after the president's lawyer filed a complaint saying Republican People's Party (CHP) leader Kemal Kılıçdaroğlu insulted Erdoğan on Tuesday.
The president’s lawyer, Hüseyin Aydın, posted the letter of complaint on his official Twitter account on Dec. 6. The letter accuses Kılıçdaroğlu of voicing statements "that are part of a perception operation that the Fethullahist Terrorist Organization [FETÖ] has long been carrying out against our president."
"The statements of the accused and the political party that he leads about our president overlap with the discourse of FETÖ," the letter says. The network of Islamic preacher Fethullah Gülen, referred to as the FETÖ by Ankara, was one of the masterminds of last year's coup attempt.
The CHP leader accused Erdoğan of "ignoring corruption" and said he would be brought to account for his mistakes, remarks which were also quoted by Aydın in his complaint.
He said that Turkey's National Intelligence Organisation (MİT) had in April 2013 warned Erdoğan that Iranian-Turkish gold trader Reza Zarrab was breaking US law and this could damage Turkey. Zarrab, once close to the government and Erdoğan, is now testifying as a star witness in a potentially explosive New York trial against a Turkish banker. Zarrab implicated the president in a scheme to subvert US sanctions against Iran.