The ruling that the rights of Yuksel Yalcinkaya had been violated could create a significant precedent, with thousands of similar cases pending before the Strasbourg-based court.

Turkey blames a group led by the US-based preacher Fethullah Gulen for being behind the failed attempt to oust President Recep Tayyip Erdogan, alleging that a messaging app called ByLock was used to coordinate the plot.

Thousands of people were arrested and convicted in the wake of the coup on charges of links to the group, even though Gulen has denied any involvement.

The ECHR, in a damning ruling, said "anyone who had used ByLock could, in principle, be convicted on that basis alone of membership of an armed terrorist organisation".

Meanwhile, there were "procedural shortcomings" in the trial for Yalcinkaya, who was arrested in 2016 and convicted in 2017, that infringed on his right to a fair trial, it added.

Turkey considers Gulen's organisation to be a terror group and has repeatedly called for the preacher -- a former Erdogan ally -- to be extradited from the United States.

The ECHR said there were currently 8,500 applications of a similar nature and "given that the authorities had identified around 100,000 ByLock users, many more might potentially be lodged".

It said that the failure of the Turkish judiciary concerning evidence of the use of ByLock was "systemic" and that Turkey had to take action to tackle these "systemic problems."

This was a final verdict issued by the Grand Chamber of the ECHR, whose rulings are binding for all 46 members of the Council of Europe.

But Ankara has flouted a succession of rulings by the court in recent years, notably concerning two anti-Erdogan figures, the Kurdish politician Selahattin Demirtas and philanthropist Osman Kavala, who remain in jail despite ECHR rulings that their rights were violated.

Turkey's Justice Minister Yilmaz Tunc denounced the court's ruling in a post on X.

"It is unacceptable that the ECHR oversteps its authority and issues a ruling... on a case that our judicial authorities, at every level... have determined there is sufficient evidence," he wrote.