An imprisoned co-leader of Turkey’s main pro-Kurdish party is planning a hunger strike to protest “inhumane” conditions of the jail where he has been held.

Axar.az reports that the Peoples' Democratic Party (HDP) said Thursday that Selahattin Demirtas would begin his strike at the prison in the northwestern province of Edirne on Friday.

The party said the action was meant to protest horrible conditions in the facility and because officials there had refused to discuss what Demirtas has termed “unlawful practices.”

The HDP, the second largest opposition party in Turkey, has been a main critic of plans by the Ankara government to change the country’s constitution. The party has campaigned against the April 16 referendum on approving the changes.

If approved, the amendments proposed by the ruling Justice and Development Party would usher in an executive presidency, which would bolster President Recep Tayyip Erdogan's powers.

Erdogan says the proposed constitutional changes would guarantee political stability, arguing the new system would prevent the return of fragile coalition governments led by the prime minister. The post of prime minister will be abolished in the new system.