Turkish prosecutors on Friday demanded life terms for 28 suspects accused of setting up bugs to eavesdrop on President Recep Tayyip Erdogan and other top officials.

The case is one of the key pillars in Erdogan's offensive against followers of US-based preacher Fethullah Gulen, who he accuses of setting up a parallel state aimed at ousting him from power.

The suspects have been charged with espionage, "seeking to disrupt the unity and integrity of the state" and illegal use of communications, the official Anatolia news agency reported.

They stand accused of setting up bugs to eavesdrop on Erdogan, Prime Minister Ahmet Davutoglu, intelligence chief Hakan Fidan, Turkey's top general Necdet Ozel, and other senior figures.

The 28 include former top managers of Turkey's telecommunications agency TIB and its technology agency TUBITAK.

Friday's announcement follows the completion of a month-long investigation, Anatolia reported.

The prosecutors are demanding aggravated life terms for the accused, meaning the prisoners would be held in a single cell with limited contact with other prisoners.

The prosecutors have now sent the case to the prosecutor general in the capital, Ankara,ahead of the trial.

The case is linked to the corruption scandal that rocked Erdogan's government and his inner circle in December 2013, and was partly based on intercepted telephone conversations.

The bugs are alleged to have been laid in the period up to 2014 when Erdogan was prime minister and Davutoglu his foreign minister.

Leaked tapes emerged in February 2014 of Erdogan allegedly telling his son Bilal to dispose of some 30 million euros ($37 million) in cash. Erdogan has dismissed the recordings as a "vile montage".

Since becoming president in August 2014, Erdogan has set out to crush Gulen, who gained huge influence in Turkey through his supporters in the police and judiciary as well as a system of private crammer schools.

Gulen's supporters argue that the drive against his movement is a crass violation of rights and a sign Turkey is headed down an authoritarian path under the rule of Erdogan.