The trial opened at an Ankara court on Friday of 13 former Turkish police officers accused of setting up bugs to eavesdrop on President Recep Tayyip Erdogan while he was prime minister.
The defendants are accused of carrying out and aiding political espionage as well as the violation of the privacy of public officials, the official Anatolia news agency reported.
If found guilty, they could be jailed for up 36 years and six months, it added. Erdogan's own lawyers are taking part in the closed court hearing.
The trial is linked to the sensational corruption scandal of December 2013 that rocked the government of Erdogan as well as his inner circle and was partly based on intercepted telephone conversations.
Erdogan denied the claims, saying the graft allegations had been concocted by his arch foe Fethullah Gulen, who lives in self-imposed exile in the United States, to bring down his government.
According to the indictment, the police are accused of placing bugs in various locations used by Erdogan while he was prime minister in 2011.
The devices are said to have been placed inside electricity plugs.
According to the Hurriyet newspaper, five of the police are still on the run, meaning only eight of the suspects were able to appear in court.
One of the accused is Zeki Bulut, who was Erdogan's former chief bodyguard.
The police appear to have been arrested during the waves of detentions in 2014 of police deemed loyal to Gulen.
The authorities in December also raided a newspaper and television channel with close links to Gulen, provoking a major rift with the European Union over an erosion of media freedoms.
Leaked tapes emerged in February 2014 where Erdogan allegedly told 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, whom he accuses of setting up a parallel state through supporters in the police and the judiciary.
The start of the trial comes as a Turkish parliamentary commission is on January 5 set to decide whether four ex-cabinet ministers who resigned in the wake of the corruption scandal should go on trial.