Turkey's main opposition parties, lawyers and rights groups demanded the release of Kurdish leader Selahattin Demirtas on Tuesday following a binding European rights court ruling nine years after his imprisonment.

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 jailed after helping briefly break President Recep Tayyip Erdogan's grip on parliament.

The ruling came as Ankara pursues peace with Kurdish PKK militants who have renounced their four-decade armed rebellion and are seeking to engage in a democratic political struggle for Kurdish rights.

In a handwritten letter, a copy of which was posted on X, Demirtas thanked all those who spoke out in favour of his release and expressed his support for the peace process.

"While we strive together to open a brand-new page, if we remain stuck on the mistakes of the past, we risk compromising our future. We will resolve all remaining issues together within the scope of democratic politics. But first and foremost, we will strive for peace," he wrote.

Last year, Demirtas was handed a 42-year jail term 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 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 with Sezgin Tanrikulu, its lawmaker in Diyarbakir, the main city in Turkey's Kurdish-majority southeast, agreeing the decision "must be implemented immediately".

There was no immediate reaction from the Turkish authorities to the ruling.

- 'Very rare' -

Turkey's Human Rights Association (IHD) said the 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 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 demanded his immediate release.

Close Erdogan ally Devlet Bahceli, who heads the nationalist MHP party, told Turkish media Demirtas had "achieved a result through legal means" adding: "His release will be a positive step for Turkey."

In his letter, Demirtas thanked Bahceli, hailing his stance which had "bravely shattered taboos and showed that peace cannot be built by surrendering to fears".