Western diplomats packed a Turkish courtroom on Monday to observe the latest stage of a trial of a civil society leader whose detention has set off a confrontation with Europe's top human rights body.

Businessman and philanthropist Osman Kavala, who has been held without conviction since October 2017, has become a symbol to supporters of the purges President Recep Tayyip Erdogan unleashed after surviving a coup attempt in 2016.

The Turkish leader accuses the 64-year-old of financing a wave of 2013 anti-government protests and playing a role in the coup plot.

An appeal last October from 10 Western countries -- including the United States and major European powers -- for Turkey to release Kavala stirred up a diplomatic standoff that nearly saw Ankara expel their ambassadors.

The Council of Europe has given Turkey until Wednesday to either free Kavala or "submit in concise form" its justification for detaining him.

The Strasbourg-based rights body could decide on February 2 to move ahead with rare disciplinary proceedings against Ankara.

An AFP reporter in court on Monday saw diplomats representing the European Union and nine Western countries attend the latest stage of the marathon trial in Istanbul.

The European Court of Human Rights (ECHR) first ordered Kavala's release in 2019, saying that he was being held to silence his defence of human rights.

Ankara's failure to comply with Wednesday's deadline could see the Council decide to send the case back for further action to the ECHR.

The lengthy infringement procedure could ultimately see Turkey lose its voting rights or even get kicked out of the pan-European rights body it first joined in 1950.

Turkey's foreign ministry said it views the Council's actions -- only launched once before against any of its 47 member states -- as "interference" in domestic Turkish affairs.

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

Human Rights Watch last week warned that Erdogan "has set back Turkey's human rights record by decades," highlighting the country's withdrawal last year from a convention protecting women against domestic violence and singling out Kavala's case.