Demirtas -- a once prominent and charismatic leader within Turkey's Kurdish movement -- was arrested on November 4, 2016, and jailed on suspicion of "terror-related" offences at the height of his political career.

Despite the years spent in jail, the 52-year-old still commands fierce loyalty for his outspoken support of the Kurdish minority and their rights.

Western governments see him as a political prisoner who was jailed after helping briefly break President Recep Tayyip Erdogan's grip on parliament.

Last year, he was handed 42 years for his alleged role in the deadly 2014 protests that erupted when Islamic State group jihadists overran the Syrian town of Kobane.

He has also been charged with other crimes including "terrorism"-related offences linked to the Kurdish militant PKK and insulting the president.

The European Court of Human Rights (ECHR) has repeatedly demanded his release, denouncing his arrest and trial as unfair and unlawful in a string of rulings ignored by Ankara.

The court issued its final, definitive ruling on Monday night, confirming that his rights had been violated and ordering his release.

Tuncer Bakirhan, co-chair of the pro-Kurdish DEM, parliament's third largest party, said the Turkish government must comply with the ruling and immediately release him.

"The ECHR clearly determined that (Demirtas') detention in the Kobane case was politically motivated and demanded his release," Bakirhan told a party gathering.

"Turkey is obligated, under Article 46 of the ECHR, to comply with this and previous rulings" so Demirtas and his jailed former co-chair Figen Yuksekdag, 53, "must be released immediately, without delay", he said.

The call was echoed by the main opposition CHP party.

Sezgin Tanrikulu, its MP in Diyarbakir, the main city in Turkey's Kurdish-majority southeast, said the ECHR decision was "now final.. (and) must be implemented immediately".

- 'Very rare' -

In a post on X, Turkey's Human Rights Association (IHD) said the ECHR ruling confirmed there were no legal grounds for the arrest of Demirtas and others, which it described as "premeditated political decisions".

It demanded his "immediate release, along with all political prisoners held in jail in violation of the law".

Erinc Sagkan, head of the Turkish Bar Association said the ruling found Turkey to be in breach of article 18 of the European Convention on Human Rights -- which relates to detention for political purposes -- in a step it described as "very rare".

"It tells us there was an 'abuse of power' in the trial process, that political interventions were made in the proceedings and that the fundamental pillars of the procedural structure -- starting with the guarantees of impartiality and an independent tribunal -- were fundamentally eroded," he wrote on X.

"The essential matter now is to implement the requirements of the ECHR decision without delay."

One of Demirtas's lawyers said they had filed court papers demanding his immediate release.

Adding to the sense of optimism, Devlet Bahceli, head of the nationalist MHP party and a close Erdogan ally, appeared to back the release calls.

"Mr Demirtas has achieved a result through legal means. His release will be a positive step for Turkey," Bahceli said in remarks broadcast by Turkish media.