By Jonny Dymond BBC correspondent in Istanbul

Former PKK leader Ocalan is serving life in prison

His lawyers claim he was treated inhumanely and denied a fair trial after he was captured by Turkey's intelligence services in 1999.

Mr Ocalan's capture and conviction were a cause for huge jubilation across much of Turkey.

He was held responsible by many for thousands of deaths in the 15-year war between the PKK and the Turkish security forces.

Long process

But Mr Ocalan was tried under a system of justice that has now been abolished in Turkey and under circumstances that the European Court clearly feels warrants further investigation.

In particular, the presence of a military judge at his trial, his lack of access to the proceedings of trial, his detention prior to his trial and the imposition of the death penalty after his trial have all been criticised in a previous judgement of a lower chamber of the European Court.

This is just the beginning of what could be a long process.

But previous judgements of the court in matters pertaining to Turkey have been hostile, in particular to the presence of a military representative amongst the judges.