The European Court of Human Rights ruled on Tuesday that Turkey violated the right of the former head of the pro-Kurdish opposition, Selahattin Demirtas, to a speedy trial.
Demirtas, one of Turkey's best-known politicians, was sentenced in September to more than four years in jail on terrorism charges, after having been detained for two years while on trial.
"The court found that the judicial authorities had extended Mr Demirtas' detention on grounds that could not be regarded as 'sufficient' to justify its duration," the Strasbourg-based court said in a statement.
During the country's presidential elections this year, Demirtas campaigned against Turkey's President Recep Tayyip Erdogan, appearing on state television from his jail cell.
Mr Erdogan won the elections and Demirtas came third with 8.4 per cent of the vote.
The court also criticised the Turkish government for keeping Demirtas in jail during to election and a referendum on changing the constitution to give the president more powers.
"The extensions of Demirtas' detention, especially during two crucial campaigns … pursued the predominant ulterior purpose of stifling pluralism and limiting freedom of political debate," it said.
But the court ruled against some of Demitras' complaints, including the allegation he had not been detained on reasonable suspicion of having committed a criminal offence.
Demirtas was awarded €10,000 in (Dh42,000) in damages and €15,000 in costs.
The Kurdish opposition leader was accused of having links to the outlawed Kurdistan Worker's Party (PKK), which is considered a terrorist organisation by Turkey, the EU and the US.
Two months ago a court sentenced Demirtas to more than four years in jail on terrorism charges related to a speech he gave in 2013. He faces several other terrorism-related charges and a total of 142 years in prison if convicted.
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