Turkey's top court on Tuesday signed off on the controversial three-year detention of rights activist Osman Kavala in a case watched closely for signs of a political thawing under President Recep Tayyip Erdogan.

The internationally recognised patron of culture and the arts is still in prison despite having been acquitted in February in connection with widespread protests that posed the first serious challenge to Erdogan's rule in 2013.

He was re-arrested before he could walk free and jailed on fresh charges of espionage and involvement in a failed 2016 coup that was followed by mass arrests and a sweeping crackdown on political dissent.

Constitutional Court judges ruled by an 8-7 margin that the 63-year-old's continued detention did not violate his rights to liberty and security. Their formal reasoning is expected to be published in one or two months.

Kavala called the ruling politically motivated and "extremely worrying" for Turkey's rule of law.

"That the court found lawful accusations against me which are not based on any evidence is beyond all reason," he said in a statement released from his prison on the outskirts of Istanbul.

The Parisian-born businessman would be sentenced to life in prison if convicted of trying to overthrow the constitutional order and 20 years if found guilty of espionage.

His next court hearing is scheduled for February 5.

Kavala was a founding member of philanthropist George Soros' Open Society Foundation in Turkey and headed Anadolu Kultur -- a group that promoted cross-cultural ties through the arts at the time of his arrest.

He rejected the new charges in a court appearance this month by arguing that he opposed military coups and had "criticised the army's interference in politics".

Erdogan's critics are increasingly worried about what they say is Turkey's practice of arbitrary detentions and abusive prosecutions of everyone from journalists and lawyers to the government's formal political foes.

Kavala has remained in prison despite being acquitted in February in connection with 2013 anti-government protests.
Kavala has remained in prison despite being acquitted in February in connection with 2013 anti-government protests. AFP / OZAN KOSE

These add to a litany of problems in Turkey's relations with the European Union that have effectively frozen Ankara's process of joining the bloc.

Emma Sinclair-Webb of Human Rights Watch denounced Tuesday's ruling as "another nail in (the court's) own coffin". It came in "flagrant disregard" for the European Court of Human Rights' (ECHR) order to release Kavala in December 2019, she added.

"Serving political masters gets you further than worrying about law in Turkey," she told the court's judges on Twitter.

Although Turkey is bound by ECHR rulings it has repeatedly ignored them under Erdogan.

Kavala has the distinction of drawing Erdogan's wrath without ever having run for public office.

The Turkish leader branded him the "red Soros of Turkey" when he was first detained after landing at an Istanbul airport from Gaziantep in the southeast in October 2017.

Erdogan pledged last month to push through judicial reforms that could improve Turkey's investment climate during its deep economic malaise.

But he also took aim at Kavala and made clear that he still held him responsible for the 2013 unrest.

"We can never be with financiers of the (2013) events," Erdogan said. "We can never be with Kavalas."

US academic Henri Barkey is being tried in absentia alongside Kavala in the new case.

The charge sheet alleges Barkey used a conference he attended on an island off Istanbul as cover to coordinate the putsch with Kavala.

Barkey told AFP this month that the charges against him were "a travesty of the first order".

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