The Grand Chamber of the European Human Rights Court ruled on Thursday that international treaties had been breached in the 1999 trial of Abdullah Ocalan.
In particular, they ruled that the Kurdish Workers Party (PKK) leader had been treated inhumanely by being transferred to the prison island of Imrali, off Istanbul, six years ago.
The court said he had been denied the right to a fair trial, while his legal representatives had been barred from contacting him for a period after he was detained.
In addition, the State Security Court that tried Ocalan contained a military judge, a procedure breaking European norms and since abolished by Turkey - along with the security court itself.
In its response, Turkey has said it will do "what it has to do" after the European Court of Human Rights ruled that the 1999 trial of Abdullah Ocalan was unfair.
The announcement on Thursday signalled Ankara's readiness to retry the Kurdish rebel leader.
Speaking on TRT state television, government spokesman Cemil Cicek also told the Turkish people there was no need to fear Ocalan's release and said they should trust the state and its judicial organs to handle the case properly.
However, the European court's decision will likely cause a nationalist backlash in Turkey, although the country's leaders have appealed for calm.
There has been a resurgence in
PKK violence recently
Ocalan was captured by Turkish Special Forces commandos in Nairobi, Kenya, where he had been hiding out at the Greek embassy, in April 1999.
After trial, he was sentenced to death, although this was later commuted to life imprisonment when Turkey abolished the death penalty as part of reforms aimed at bringing the country into the European Union.
Turkey is a member of the Council of Europe and thus legally obliged to act on European Human Rights Court decisions, although in this case, it is not legally bound to order a retrial. However, it is likely to do so or risk the wrath of prospective EU partners.
Yet many Turks react strongly against the idea of retrying a man whom many regard as almost solely responsible for two decades of fighting.
Since 1985, despite periodic ceasefires, the PKK and the Turkish army have fought - mainly in the country's poverty-stricken southeast - with more than 30,000 killed in the conflict.
Turkey may risk the wrath of EU
partners if it ignores the ruling
Fighting has also recently been resurging. This week alone saw three suspected PKK members killed by security forces in a clash near Tunceli, 800km east of Ankara.
Government ministers, aware of how high feelings could run - particularly after a recent wave of nationalism following an alleged Turkish flag-burning incident by Kurds in the southeast last month - have been trying to play things down.
"There are hundreds of cases at the European court," Dengir Mir Mehmet Firat of the ruling Justice and Development Party (AKP) said on Wednesday.
"The Turkish Republic is a state based on the rule of law and will undertake the procedures that the law requires," Firat said.
"If the rest of the world wants to review the case of a terrorist, the Turkish judiciary is independent and Turkey is a transparent state of law," he added.
Many Turks feel EU demands,
especially on Cyprus, are unfair
Justice Minister Cemil Cicek also warned against political groups trying to use the decision to attack the administration.
"If some people see this as a way to attack the government," he said on Wednesday, "it would be Turkey cornered in the end, not the government."
A senior legislator from the governing Justice and Development Party (AKP), Sadullah Ergin, immediately branded the ruling "undesired," while the party's deputy chairman said Turkey would take steps to fulfil its obligations.
Demands and no actions
A nationalist reaction is being widely predicted. "There is bound to be some sort of backlash," Iltar Turan of Istanbul's Bilgi University's international relations department said.
"If the rest of the world wants to review the case of a terrorist, the Turkish judiciary is independent and Turkey is a transparent state of law"
Dengir Mir Mehmet Firat,
Justice and Development Party member
"In particular, when you look at relations with the European Union, recently. It seems that what has come from the Europeans has been demands and no actions," he said.
"Turks are particularly frustrated that nothing has been done, for example, to end the isolation of the Turkish Cypriots."
Opinion polls suggest that the rightist National Action Party - all but written off after the 2002 general election - is now riding high, with many other parties "playing the nationalist card", Turan said.
What effect this is likely to have on Turkey's estimated 12 million ethnic Kurds is a matter of some concern to Kurdish organisations and individuals.
"We are worried, of course," Hasan Saib, an ethnic Kurdish cafe owner in the Istanbul district of Gunesli, said. "After the flag incident last month, we were tense, waiting to see what might happen. Now, it's the same again."
Ankara on Thursday hinted that
it may retry the Kurdish leader
Now, the retrial decision, which cannot be appealed, will go to the Ministerial Committee of the European Council for final ratification, most likely next month. Ankara must then decide what action to take.
"I think the government will convince people that it needs to go to retrial," Istanbul-based lawyer Sema Halkili said.
"No one really thinks a second court hearing will come up with a decision different from the first, and most of the documentation is already done, so a new trial could be conducted speedily and end up most likely with the same result."