Nearly 500 suspects, including the former head of the air force and pilots, entered a court outside Ankara on Tuesday in one of the main trials against those accused of leading last year's failed coup attempt.

In all, 486 suspects face charges including murder, membership of a terrorist organization, attempting to assassinate President Recep Tayyip Erdogan, trying to overthrow parliament and damaging public property, according to state-run media Anatolia Agency.

If convicted, most of the accused face life sentences in solitary confinement with no possibility of parole.

The "number one" defendant is Muslim cleric Fethullah Gulen, the leader of the Islamic movement that Turkish authorities blame for the failed July 15, 2016 coup attempt. 

Gulen, who has lived in exile in the US state of Pennsylvania since 1999, denies giving any orders for his followers to carry out the coup.

Coup headquarters

Of the suspects, 461 are currently in prison, 18 have been released under supervision and seven are fugitives. Many of the suspects in the trial also face charges in other coup-related trials.

Video on CNN Turk showed about 40 of the defendants being brought to the court outside Ankara built especially for the coup trials, each suspect handcuffed and accompanied by two gendarme police and flanked by an armed solider.

Crowds held signs and yelled outside the heavily guarded prison complex in Sincan demanding the death penalty for the suspects.  The death penalty has been banned in Turkey since 2004, but Erdogan has said he supports bringing it back. 

The suspects are accused of running the coup from Akinci air base, the alleged coup headquarters from where F-16 jets flew low over Ankara sending out sonic booms before later bombing parliament and other government buildings. The air base was also bombed by the Turkish air force as it put down the coup attempt. 

Coup perpetrators' identities unclear

Chief of the General Staff General Hulusi Akar and other commanders were held hostage at the base during the coup, which killed nearly 250 people.

Another key figure in the trial is former air force chief Akin Ozturk, who also faces charges in other coup related trials. He has denied the coup charges. 

The trial also brings charges against so-called "civilian imams," shadowy non-military figures alleged to be key coordinators between Gulen and the putschists.

The chief "civilian imam" is alleged to be theology academic Adil Oksuz, who was arrested at Akinci air base once the coup bid collapsed.  Oksuz was released shortly after the coup by an alleged Gulenist judge, who himself has since been arrested, and is now one of Turkey's most wanted fugitives.

Another "civilian imam" on trial is businessman Kemal Batmaz, said to be Oksuz's assistant.

The trial is expected to last 29 days.

Many questions surrounding coup

Erdogan's critics accuse the president of enabling the Gulen - one of his former allies - to infiltrate key state institutions before the two began to clash in 2013 in an open power struggle. 

Read the background the Erdogan-Gulen alliance and power struggle

Much remains mysterious about how failed coup attempt unfolded. 

One likely scenario is that Gulenist officers carried out the coup, which was then joined by anti-government officers and soldiers believing they were simply following orders.

Under this scenario, non-Gulenist officers would not have known that the coup was led by Gulenists. Traditional secular-nationalists within the military had always been weary of the Gulen movement, due to previous bogus coup trials against officers led by Gulenist prosecutors with the support of Erdogan to weaken the military.

However, Turkish authorities have yet to present any evidence Gulen gave a direct order. 

EU intelligence and many Turkey analysts have suggested suspected Gulenist officers were prompted to carry out the coup a month before a High Military Council meeting that was expected to dismiss many Gulenist officers.

It also remains unclear at what point intelligence chief Hakan Fidan, Erdogan and other senior figures learned about the coup. Some analysts and the opposition have suggested Erdogan allowed a "controlled coup" that could be put down in order to later strengthen his authoritarian rule through a second "palace coup" implemented through a state of emergency. 

More than 50,000 people have been arrested and at least 120,000 more dismissed from their jobs under a state of emergency in effect since the coup attempt. Only a small fraction of those arrested had a direct role in the coup attempt. The mass purges have raised concerns over human rights and the rule of law.