Ocalan spearheaded a brutal insurgency that has killed tens of thousands in the PKK's fight for independence and, more recently, broader autonomy in Turkey's mostly Kurdish southeast.

Founded in 1978, the Marxist-inspired group is regarded as a terror organisation by Turkey and most of its Western allies, including the United States and European Union.

The 75-year-old former guerrilla is seen as a hero by many Kurds, who refer to him simply as "Apo" (uncle in Kurdish).

Turks prefer to call him "bebek katili" (baby killer) for his ruthless tactics, which included bombings of civilian targets.

Detained 25 years ago in a Hollywood-style operation by Turkish security forces in Kenya after years on the run, Ocalan was sentenced to death.

He escaped the gallows when Turkey abolished capital punishment in 2004 and is spending his remaining years in an isolation cell on the Imrali prison island south of Istanbul.

Ocalan's family has been allowed just five visits since 2014 and his last permitted phone call came in March 2021.

"We know that he suffers from numerous chronic ailments but we have no idea what his current state of health is," lawyer Ibrahim Bilmez, whose team was last able to reach Ocalan in August 2019, told AFP.

- Hopes for peace -

Yet for a few years up to 2015, Ocalan was engaged in talks approved by President Recep Tayyip Erdogan, then premier, aimed at finding a solution for what is often called Turkey's "Kurdish problem".

Seen as the world's largest stateless people, Kurds were left without their own country when the Ottoman Empire collapsed in the wake of World War I.

Also spread across Syria, Iran and Iraq, their biggest numbers are in Turkey, where Kurds make up around a fifth of the population.

For hardline nationalists who support the post-Ottoman idea of "Turkishness", the Kurds simply do not exist.

And by no means do all Kurds back the ideas, let alone the methods, of the PKK. Erdogan has in the past been able to count on substantial levels of support in some Kurdish areas.

Ocalan's indirect talks -- led by Erdogan's former spy chief and current foreign minister Hakan Fidan -- raised hopes of an end to the PKK's insurgency and an equitable solution that would ensure the rights of Kurds within Turkey's borders.

- Waning influence -

But those ambitions were shattered in June 2015, when the PKK ended the ceasefire following a deadly suicide bombing against pro-Kurdish demonstrators, attributed to Islamic State (IS) group jihadists.

The PKK accused the government of collaborating with IS and resumed its campaign of violence with a vengeance. The military responded with crushing force, including air strikes on the PKK's rear command bases in neighbouring Iraq.

Turkey's recent wide-scale use of combat drones has pushed most of the Kurdish fighters across the border into Iraq and Syria, where Ankara continues to conduct low-scale operations that cause occasional frictions with its Western partners.

The Turkish government has defended the de facto silencing of Ocalan by saying he failed to convince the PKK of the need for peace, raising doubts about how much sway he now has over the group.

The limits of Ocalan's political influence became apparent during Istanbul's controversial mayoral election in 2019, when the pro-Kurdish party rebuffed his call not to back Erdogan's main rival in a re-run ballot.

Kurdish political leaders in Turkey are marking the 25th anniversary of his 1999 capture with a series of small marches they have been holding across Turkey since the start of the month.

- Captured in exile -

Ocalan was born into a peasant family in Omerli, a village in Turkey's southeast, on April 4, 1948. His family is a mix of Turkish and Kurdish, and his mother tongue is Turkish.

He became a leftwing militant during his time studying politics at university in Ankara, a period shadowed by the spectre of military coups.

He spent the years ahead of his arrest on the run and in the early years of his exile he was given refuge in Syria, from where he led the PKK's armed struggle, causing major friction between Damascus and Ankara.

Political pressure forced him out in 1998 and with the net closing in, Ocalan raced from Russia to Italy to Greece in search of a haven.

He ended up in the Greek consulate in Kenya, where US agents rapidly got wind of the presence of the most wanted man of its ally Turkey and tipped off Ankara.

Lured into a vehicle and told he would be taken to the airport to travel to the Netherlands, Ocalan instead was handed into the arms of Turkish army commandos and taken on a private plane to face trial at home.