A Turkish court ordered leading activist Osman Kavala to remain in prison Monday, amid fresh calls for his release in the high-profile case that has drawn widespread rebuke.

The 64-year-old philanthropist has been held without conviction since October 2017, accused of financing 2013 anti-government protests and playing a role in a coup plot against President Recep Tayyip Erdogan.

The European Court of Human Rights (ECHR) has called for Kavala's release, which Turkey has refused to comply with.

This month, the Council of Europe (COE) launched disciplinary action over Turkey's failure to free him.

A three-judge panel on Monday refused to release Kavala and set the next hearing for March 21.

Kavala did not appear in court, and his lawyers questioned the tribunal's impartiality.

"Kavala is not being tried in this tribunal, but in political party meetings," defence lawyer Tolga Aytore told the court.

Western diplomats, including from France and Germany, attended the hearing on Monday, according to an AFP reporter.

The COE ruled this month that Turkey had failed to comply with a 2019 ruling by the ECHR to release Kavala.

Under the rules of the Strasbourg-based COE, the case has been referred back to the ECHR, which will examine if Turkey has complied with its 2019 ruling.

Turkish activist Osman Kavala has been held without conviction since October 2017
Turkish activist Osman Kavala has been held without conviction since October 2017 Anadolu Culture Center via AFP / Handout

Turkey has been a member of the COE since 1950 and is party to the European Convention on Human Rights.

It denounced the COE's decision as "interference" in domestic court proceedings.

The COE's verdict could prompt action against Ankara from the committee of ministers, including suspension of Turkey's voting rights or even expulsion from the body.

Ahead of the hearing on Monday, the European Parliament's Turkey rapporteur slammed Ankara for its apparent refusal to comply with the ruling.

"It's not easy to understand what the rationale is of the Turkish authorities, simply not complying with the court ruling," Nacho Sanchez Amor told AFP.

"This is not about any kind of interfering from abroad, this is about the Turkish constitution, the European Court of Human Rights is part of the judiciary system of Turkey."

He added that Turkey's refusal to comply with the court ruling would "damage the image of the country".

Erdogan has repeatedly accused Kavala of being an agent of George Soros, a billionaire financier and pro-democracy campaigner.

Kavala's supporters view his plight as a symbol of the purges Erdogan unleashed after the coup attempt, and his case has become a growing irritant on Turkey's complex ties with the West.

Government critics say Turkey's standoff with the COE underscores the profound erosion of human rights under Erdogan's two-decade rule.