Kavala, 66, was a relatively unknown patron of culture and the arts until his detention upon arrival in Istanbul from Gaziantep in Turkey's southeast in October 2017.

Still reeling from the shock of a failed but bloody coup attempt the previous year, Erdogan branded the Paris-born philanthropist as a foreign-funded agent who was scheming to topple Turkey's Islamic conservative government.

Kavala was placed in pre-trial detention and then faced a series of alternating charges that, for most Western governments, came to symbolise Turkey's drift away from democratic norms.

He was ultimately jailed for life last year in connection with a wave of 2013 anti-government protests that posed the first serious challenge to Erdogan's increasingly dominant rule.

Turkey's top appeals court upheld the conviction in September, ignoring repeated orders from the European Court of Human Rights for Kavala's immediate release.

Kavala's fate now rests in the hands of Erdogan, who could potentially pardon him -- something the president has given no indication he might do.

But responding to AFP questions through his lawyer from his jail cell on the outskirts of Istanbul, Kavala sounded defiantly upbeat.

"I have no doubt that I will be released, since I have not engaged in any activity considered a crime under the law," he said in written comments.

"What I don't know is when this will happen."

- Watching TV, feeding sparrows -

The Council of Europe has launched infringement proceedings against Turkey over its treatment of Kavala, underscoring the diplomatic sensitivity of the case.

It also awarded its top human rights prize to Kavala last month, drawing condemnation from Ankara, which accused the continent's top rights body of pursuing a "political agenda".

Erdogan has displayed repeated irritation with the West's concern about Kavala's fate.

He threatened to expel the US and nearly a dozen other Western ambassadors who signed an open letter calling for Kavala's release in 2021.

Kavala receives regular updates about the ongoing diplomatic efforts from his lawyer, who visits him two or three times a week.

He also watches news on TV, which is mostly controlled by Erdogan's allies and devotes little attention to the opposition.

To maintain his spirits, Kavala reads, writes, and takes occasional walks in the prison courtyard when the guards allow.

"I take a walk and leave bread crumbs for the sparrows," he said.

"In the afternoon, I read books and write a little. In the evenings, I watch movies" on state TV.

He tries not to think about when he might be released "in order to protect my mental health," he said.

- 'Vital for democracy' -

Kavala said it was important for the international community to keep up the fight for Erdogan's jailed rivals' release.

Erdogan unleashed sweeping purges in the wake of the coup attempt that saw tens of thousands of people either jailed or stripped of government jobs.

Turkish courts have also jailed dozens of top Kurdish politicians, lawyers, reporters and rights campaigners on "terrorism" and other loosely defined charges.

Kavala called on foreign politicians to "continue to remind the Turkish government that compliance with (European Court of Human Rights) ruling is necessary for the European legal system."

His case is also starting to make political waves in Turkey, despite the opposition's shrinking access to the media and the ruling party's dominance over most facets of public life.

Secular opposition leader Kemal Kilicdaroglu, who pushed Erdogan to a runoff in a bitterly-contested May presidential election, visited Kavala in jail for the first time last month.

Kilicdaroglu had vowed to release all "political prisoners" if elected.

"I expect them to keep this issue, which is vital for democracy, on the agenda," Kavala said, referring to Turkish opposition and international support for his cause.