The Ankara chief prosecutor's office began the probe after the president's lawyer Huseyin Aydin filed a complaint saying Republican People's Party (CHP) leader Kemal Kilicdaroglu insulted Erdogan on Tuesday, state-run news agency Anadolu reported.

The accusations are punishable by up to four years in prison.

In the complaint shared by Aydin on Twitter, the lawyer quoted extracts from a speech by Kilicdaroglu in which he said Erdogan had not left Turkey "in peace".

"If you are searching for a traitor to the people, that person at the top is sitting in the palace," Kilicdaroglu said in the speech in Ankara.

The CHP chief accused Erdogan of "ignoring corruption" and said he would be brought to account for his mistakes, remarks which were also quoted by Aydin in his complaint.

He said that Turkey's National Intelligence Organisation (MIT) had in April 2013 warned Erdogan that Turkish-Iranian gold trader Reza Zarrab was breaking US law and this could damage Turkey.

Zarrab, once close to the government and Erdogan, 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.

- Thousands prosecuted -

The complaint against Kilicdaroglu comes after Erdogan last month sued him over claims that members of the president's family, including his brother, transferred around $15 million to Bellway Limited, based in the low-tax British Crown Dependency of the Isle of Man.

Waving documents in parliament, Kilicdaroglu said he had evidence of alleged transfers made by five individuals including Erdogan's son, Ahmet Burak Erdogan, between 2011 and 2012 to the company.

But Erdogan was quick to denounce the allegations as "lies" and has repeatedly railed against the CHP as no longer the main opposition but "the party of treason".

The same prosecutor launched a probe into Kilicdaroglu for insulting Erdogan after he called him a "fascist dictator" in October.