Twenty-nine Turkish police have gone on trial in Istanbul charged with involvement in last July’s attempted coup, the city’s first trial of alleged plotters in the massive crackdown that followed the failed bid to oust president Recep Tayyip Erdoğan.

With 41,000 coup suspects under arrest and the country still in a state of emergency, the trials of the accused are expected to be the most far-reaching legal process in Turkish history.

Five months after the coup, small-scale trials of suspects have already begun in the provinces and on Monday 60 people went on trial in the south-western city of Denizli.

However, the trial in Istanbul – taking place in a huge courthouse outside the Silivri prison – is the most significant to date and the first in the Turkish metropolis.

The accused are charged with seeking to overthrow the government as well as allegedly being members of the group led by the US-based preacher Fethullah Gülen, whom the authorities accuse of leading the plot.

Gülen, whom Ankara wants to see extradited from the US, vehemently denies the charges.

The trial got under way with the reading out of the names of the accused and the judge, Fikret Demir, reading the indictment, the state-run Anadolu news agency said. This was expected to be followed by arguments for the defence. Initial hearings are expected to last until Friday. Amid tight security, special forces in camouflage gear stood guard outside the courthouse.

Of the 29 police officers due to go on trial in Silivri on Tuesday, 24 are under arrest, one is on the run and the rest are on bail. If convicted, 21 suspects each face three life sentences and the other eight officers could be handed prison terms of between seven-and-a-half and 15 years.

Those accused are alleged to have refused to protect Erdoğan’s residence in Istanbul on the night of the coup.

“We will make sure the guilty – within the framework of the law – are punished and given the most heavy punishment possible,” said Orhan Cagri Bekar, a lawyer and head of the 15 July Association, which represents victims of the coup.

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 Erdoğan’s opponents.

With the crackdown showing no sign of relenting, the interior ministry said 1,096 people suspected of links with Gülen had been detained in the past week. The coup plotters killed 248 people, according to the presidency, and Erdoğan has said there are strong public demands for retribution, even extending to reimposing the death penalty.

Erdoğan said last week that the assassination of the Russian ambassador to Ankara was carried out by a Turkish policeman loyal to Gülen, a claim not yet accepted by Moscow.

He said that allies of Gülen – who over decades built up strong networks of influence within Turkish institutions – still needed to be weeded out of the security services. “This dirty organisation is still within the military, still within the police,” Erdoğan said.

Following the start of the Istanbul trial, several others will get under way in the coming months, including of the alleged military ringleaders in Ankara. The trial of 47 suspects accused of trying to assassinate Erdoğan at a holiday resort on the Aegean Sea is due to begin in Mugla on 20 February.

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. In that case, which was strongly supported by Gülen, 275 police officers, journalists, lawyers and academics were indicted for allegedly conspiring to oust Erdoğan.

The suspects were released and their convictions quashed earlier this year, with the authorities accusing Gülen of perverting the legal process and fabricating evidence.