The hearings follow a massive crackdown on alleged putschists - 41,000 are under arrest in a national state of emergency and the trials are set to be the most far-reaching legal process in the country's history.
Five months after the coup attempt, small-scale cases involving suspects have already begun in the provinces and on Monday 60 people went on trial in the southwestern city of Denizli.
But the trial in Istanbul - in a gigantic courthouse outside the Silivri prison in the city - is the most significant to date. The government says 248 people were killed by the coup plotters.
The accused are charged with seeking to overthrow the government as well as allegedly being members of the group led by US-based preacher Fethullah Gulen who the authorities accuse of leading the plot.
Gulen, who Turkey wants to see extradited from the United States, vehemently denies the charges.
Of the 29 police on trial in Istanbul, 24 are under arrest, one on the run and the rest on bail.
Special forces in camouflage gear stood guard outside the courthouse and snipers were posted on the balconies of nearby minarets.
'Heaviest punishment'
The names of the accused and the indictment were read out, the state-run Anadolu news agency said. Arguments for the defence are expected to follow with the initial hearings expected to last until Friday.
If convicted, 21 suspects each face three life sentences on charges of staging a coup.
The other eight officers could be handed jail terms of between seven-and-a-half and 15 years on charges of "terror group" membership.
Three police helicopter pilots are specifically accused of ignoring orders to protect Erdogan's waterside residence in Istanbul. Others allegedly failed to report for duty or to support colleagues resisting the coup.
Related reading
Aleppo's Twitter girl meets Erdogan
"We will make sure the guilty - within the framework of the law - are punished and given the most heavy punishment possible," said lawyer Orhan Cagri Bekar, head of the July 15 Association, which represents victims of the coup.
President of the Turkish lawyers' association Mehmet Sari said the trials would have a wider bearing for the country.
"Never again will a group interfere in our state and to try to lead a putsch. This is what this trial will do," he said.
'Full force'
However there has been growing international alarm over the extent of the crackdown amid the state of emergency imposed after the coup, with critics concerned it has been used to target Erdogan's opponents.
With the purge showing no sign of relenting, the interior ministry said 1096 people suspected of links to Gulen were detained in the last week alone.
Erdogan has said there are strong public demands for retribution even extending to reimposing the death penalty.
He also said last week that the assassination of the Russian ambassador to Ankara was carried out by a Turkish policeman loyal to Gulen, a claim not yet accepted by Moscow.
In a rare political speech, Erdogan's prominent younger daughter Sumeyye Erdogan Bayraktar said the group of Gulen had been "unmasked" in its tactic of "taking possession of the minds and hearts" of people.
"Our state will defend the integrity, security and public will of our country with full vigour and force. All of this will be within the rule of law," Anadolu quoted her as saying in Chicago.
Following the start of the Istanbul trial, several others will start, including on February 20 of 47 suspects accused of trying to assassinate Erdogan at a holiday resort.
The courthouse in Silivri has huge resonance for Turks after it was also used in trials against suspects in 2013 accused of a separate coup plot known as Ergenekon.
That case was strongly supported by Gulen and 275 police officers, journalists, lawyers and academics indicted for allegedly conspiring to oust Erdogan. But the convictions were quashed amid accusations Gulen had perverted the process.
