Sirri Sureyya Onder, a former member of the pro-Kurdish Peoples' Democratic Party (HDP), was sentenced to three years and six months in September 2018 alongside party founder Selahattin Demirtas.

They were accused of supporting Kurdish militancy for speeches they made during the New Year festival of Newroz in March 2013.

Demirtas came third in last year's presidential election despite running from jail.

The constitutional court ruled on Thursday that the conviction against Onder was a violation of his right to free speech and ordered his release pending a retrial.

In July, the constitutional court similarly ruled unlawful convictions in a high-profile trial of academics who signed a peace petition related to the conflict against Kurdish separatists.