Erdogan said the European Court of Human Rights (ECHR) issued an "entirely political" ruling by publishing its judgement before Demirtas had used up all his legal challenges in Turkish courts.

Demirtas was convicted of disseminating terror propaganda in 2018 and is facing other charges that could see him sentenced to 142 years behind bars.

The 47-year-old led his pro-Kurdish Peoples' Democratic Party (HDP) to its first-ever appearance in the Turkish parliament in 2015 and challenged Erdogan in the 2016 presidential election.

He finished third with almost 10 percent of the vote.

Turkish authorities suspect the HDP of being a political front for outlawed Kurdish militants, who have waged an insurgency against the state since 1984.

Demirtas is also accused of fomenting 2014 protests that spread to Turkey after Islamic State (IS) group jihadists fought to capture the mainly Kurdish northern Syrian border town of Kobane.

That unrest claimed the lives of 37 people in Turkey.

The European court ruled Tuesday that Turkish authorities violated Demirtas's rights to free speech and called his jailing "cover for an ulterior political purpose".

Turkey is bound by the Strasbourg-based court's decision because it is a member of the Council of Europe.

But Erdogan called the court's decision full of "double standards" and "hypocrisy".

"The ECHR should know that they defend, they stand behind a terrorist," Erdogan said.

"He is guilty in the eyes of our nation not because of his political beliefs but because he failed to distance himself from terror groups that were responsible for the killing of dozens (of people)," he said.

Erdogan's comments came moments after the ECHR reported being the target of a "large-scale cyberattack" shortly after it published the Demirtas ruling.

The court said the attack disabled its site for several hours.

"At this time, no data appears to have been lost," the court's press agency told AFP.

The ECHR said the identity or origin of the attackers had not yet been established.