ISTANBUL (AP) — A Turkish court has formally arrested Amnesty International's chairman in the country pending trial on charges of membership in a terror organization, Turkey's official Anadolu news agency said Saturday. The rights group called it a "mockery of justice."

Taner Kilic was arrested in the western province of Izmir along with five other attorneys who are accused of using ByLock, an encrypted mobile messaging application. Kilic was detained on Tuesday and arrested late Friday.

ByLock is allegedly used by the network of U.S.-based cleric Fethullah Gulen, who Turkey blames for orchestrating last summer's coup attempt and leading a terror organization. Gulen denies all allegations.

Amnesty called Kilic's arrest a "devastating injustice" and called for his release. The rights group said authorities allege Kilic's link to Gulen by claiming that ByLock was discovered to have been on his phone in August 2014. Kilic has denied downloading or using the application and Amnesty said there was no substantiated evidence to support the claim.

Nils Muiznieks, the Council of Europe's commissioner for human rights, expressed dismay over Kilic's arrest and called for his release in the absence of credible evidence. He said the detention is part of "the worrying trend of judicial actions currently targeting human rights defenders and an increasingly wide range of other civil society actors in Turkey."

Amnesty's secretary general, Salil Shetty, described Kilic as "a principled and passionate human rights defender" who has been critical of Gulen's movement.

Shetty said the charges against Kilic are without merit and "show just how arbitrary, just how sweeping, the Turkish government's frenzied pursuit of its perceived enemies and critics has become."

Under a state of emergency declared after the failed coup, more than 50,000 people have been arrested and 100,000 public employees expelled from their posts for alleged links to Gulen.

Kilic has served on the board of Amnesty International for various periods since 2002.