Those on trial in the western city of Izmir face multiple charges over the failed July 2016 coup aimed at ousting President Erdogan.
The suspects, 152 of whom are in pre-trial detention, include ex high-ranking military officials like former Aegean Army Command Chief of Staff Major General Memduh Hakbilen, the state news agency Anadolu reported.
The "number one" suspect is US-based Islamic preacher Gulen who is charged with ordering the failed July 15 coup, an accusation he strongly denies.
Ankara also accuses the movement Gulen leads of being a "terror organisation" although the group insists it is a peaceful organisation promoting moderate Islam.
Those on trial in the western city of Izmir face multiple charges including being a member of the "armed terror group".
Turkey has repeatedly asked the United States to extradite Gulen, who has been living in exile there since 1999.
The previous administration of Barack Obama had insisted that a possibly slow legal process should take its course but Ankara is hoping for a more eager response to its extradition request from new President Donald Trump.
Other charges against the suspects include seeking to prevent parliament performing its duty as well as attempting to remove the constitutional order, the agency said.
The suspects face life imprisonment if convicted in a trial expected to take two months, the Hurriyet newspaper reported.
It said Izmir was believed to be one of the main plotting hubs of the coup with the Aegean city used as a "logistical base".
A special courtroom was built to accommodate the size of the trial and security was tight ahead of the hearing.
Intensive security measures included a drone above the courthouse, bomb-sniffing dogs and commando units.
Previous trials have opened in Istanbul and smaller cases in other provinces in what is expected to be the largest legal process in the country's history.
Some 43,000 people are under arrest ahead of trial in a large-scale crackdown within a state of emergency declared after the coup which remains in place.
In December, 29 police officers went on trial in Istanbul accused of failing to defend Erdogan while 62 rebel soldiers are being tried over claims they attempted to take over Istanbul's Sabiha Gokcen Airport on the night of July 15.