Prosecutors are seeking a total of 142 years in prison for Selahattin Demirtas, the co-chairman of the Peoples' Democracy Party - or HDP. He is charged with leading a terror organization, engaging in terror propaganda and other crimes.
The trial is one of many against Demirtas, who is imprisoned in northwestern Turkey. He did not attend Thursday's trial.
Demirtas was arrested last year when the government launched a crackdown following a failed coup attempt, which critics say has been expanded to target all government opponents and dissenting voices.
The government accuses the HDP, the nations' third-largest party in parliament, of links to the outlawed Kurdistan Workers' Party, or PKK. The HDP denies the accusation.
Prosecutors accuse Demirtas of being a leader of Turkey's outlawed Kurdish rebels and hold him responsible for street violence that broke out in 2014 during protests against government policies on Syria.
The violence led to more than 30 deaths.
The New York-based advocacy group, Human Rights Watch, criticized the trial, calling it "part of a wider pattern of repression against government critics."
"Holding the leader of a major opposition party in pre-trial detention for over a year on flimsy charges is another example of the political abuse of the criminal justice system we are repeatedly seeing in Turkey," said Hugh Williamson, Human Rights Watch's director for Europe and Central Asia.
The Turkish parliament stripped lawmakers of legal immunity last year, paving the way for the arrest of several HDP legislators - including Demirtas and former co-chairwoman Figen Yuksekdag - on terror-related charges.
The arrests follow the collapse of a fragile peace process between the state and the PKK in 2015. The group is considered a terror organization by Turkey and its Western allies.
Yuksekdag lost her seat in parliament earlier this year after an appeals court confirmed her 2013 conviction for engaging in terrorist propaganda, and later stepped down as the HDP's co-leader.
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