Selahattin Demirtas, the charismatic co-chairman of the Peoples' Democratic Party (HDP), has been behind bars for more than a year on “terror” charges and alleged links to the Kurdistan Workers’ Party (PKK).

Eight other HDP MPs are also being held, and in a new blow to the party, HDP lawmaker Idris Baluken was on Thursday sentenced to 16 years and eight months in jail for "membership of a terrorist organization" and "making terrorist propaganda".

Demirtas was the party's most successful co-leader, taking the HDP into parliament for the first time in June 2015 and again in November of that year.

"I will not stand as a candidate for co-chair at this congress, so we can meet the new period of political struggle more powerfully... (and) continue on our path with new friends and with new excitement," Demirtas said in a letter to the party from his jail cell.

The HDP said he wanted to leave because of fears his current situation would continue.

Demirtas, 44, is charged with "managing a terror organization", "making propaganda for a terror group" and "inciting criminal acts" among other accusations.

The former human rights lawyer finally went on trial on December 7 but supporters were incensed that he was not allowed to appear in court.

He faces up to 142 years in prison if convicted and under Turkish law, he would lose his membership of the party.

"We are not people of status or high office, but people of duty and responsibility," Demirtas said in the letter as he vowed to never "hold back from serving the HDP".

The party congress will be on February 11 in Ankara.

Demirtas was taken into custody in November 2016 along with a dozen HDP MPs including former co-chair Figen Yuksekdag. She was stripped of her MP status in February 2016 and stepped down in May last year.

The HDP splits its top posts between a man and a woman.

Demirtas said in the letter he was being "unlawfully held as a political hostage".

"The motive for this treatment against me can only be described as political revenge."

The HDP is accused of being a political front for the outlawed Kurdistan Workers' Party (PKK), which has waged an insurgency in Turkey since 1984.

The HDP has been hit hard with 11,500 officials, members and party sympathizers detained since the end of the ceasefire.

Reporter’s code: 50101