EU: The European Court of Human Rights has declared the 1999 trial of Kurdish rebel Abdullah Ocalan unfair.
The decision puts unwelcome pressure on the Turkish government with one eye on EU accession to order a retrial defying nationalist anger.
Turkey's centre-right government, hoping to start EU accession talks in October, said after the ruling it would do "what it has to do".
The panel of judges said in a statement read out at the court after reaching their verdict by a vote of 11 to 6: "The applicant was not tried by an independent and impartial tribunal."
The judges said Ocalan, serving a life sentence as the sole inmate of a Turkish island prison, had not had proper access to legal counsel or the facilities needed to prepare his defence.
Dozens of Kurds cheered and chanted outside the court under a banner declaring "Free Ocalan - Peace in Kurdistan".
Ocalan was found guilty in June 1999 of "treason through separatism" by a state security court, four months after he was captured in Kenya.
Such courts, used to handle cases of crimes against the state, have since been banned.
An original death sentence was commuted to life imprisonment after Turkey scrapped the death penalty in 2002, in line with EU requirements.
Speaking on TRT state television, government spokesman Cemil Cicek said Turkey would do what it had to do, but there was no need to fear Ocalan's release.
"We must be as cold-blooded as possible ... This is not the end of the world ... Our people must not be concerned [ about the possible outcome of a retrial], they must trust the state and the judiciary," he said.