Figen Yuksekdag and the other co-leader of the pro-Kurdish Peoples' Democratic Party (HDP), Selahattin Demirtas, were detained in November and have been held in jail ever since.
Their incarceration gravely alarmed the European Union and raised further concerns over use of the state of emergency imposed in Turkey in the wake of the failed coup last July.
The party in May named Serpil Kemalbay as Yuksekdag's successor as co-leader after the Supreme Court revoked her party membership. Demirtas, however, remains in his post.
Dozens of Yuksekdag's supporters crowded the courthouse, shouting "Resisting, resisting we will win!" and "Women are proud of Figen!"
Her charges include directing an armed terrorist group, making terror propaganda and inciting people to commit a crime.
Both she and Demirtas are accused of links to the outlawed Kurdistan Workers' Party (PKK) which has waged an insurgency inside Turkey since 1984.
The government says the HDP is merely the political wing of the PKK, an accusation the party denies.
The charges specifically relate to October 2014 protests in Turkey sparked by the seizure by Islamic State (IS) jihadists of the mainly Kurdish Syrian town of Kobane.
The government accused the HDP of urging people to take to the streets across Turkey in protests that left dozens dead. But the HDP blames Turkish police for the violence.
The HDP splits all its major posts, including those of municipal mayors, between a man and a woman to promote gender equality.
It is not yet clear when the trial of Demirtas -- seen as one of the few politicians in Turkey to rival the rhetorical skills of President Recep Tayyip Erdogan -- will start.
The HDP is the third largest party in the Turkish parliament but a dozen of its MPs have been arrested, in what supporters say is punishment for daring to oppose Erdogan.
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