The head of Amnesty International in Turkey has been charged with membership of a terrorist organisation and remanded in custody pending trial, the group has said.
Taner Kilic was detained on Tuesday in the province of Izmir along with 22 other lawyers.
The arrests were part of a crackdown following last July's failed coup attempt.
Amnesty called the charges "a mockery of justice".
The human rights group's secretary general, Salil Shetty, demanded Mr Kilic's immediate release and said charges against him should be dropped.
"The charges... show just how arbitrary, just how sweeping, the Turkish government's frenzied pursuit of its perceived enemies and critics has become," he said.
Amnesty's Turkey researcher Andrew Gardner tweeted: "Human rights defender, Amnesty Turkey chair Taner Kilic remanded in pre-trial detention. No credible evidence presented at hearing. Shame!"
Amnesty has been a vocal critic of the crackdown on suspected coup plotters. It said last year it had "credible reports" of detainees being subjected to "beatings and torture, including rape".
Mr Kilic is accused of using an encrypted messaging application called Bylock that the government says was used by followers of the US-based Islamic preacher Fethullah Gulen.
President Recep Tayyip Erdogan accuses Mr Gulen of instigating the coup attempt - a charge the cleric denies.
Amnesty said in a statement that Mr Kilic denied "ever having downloaded or used Bylock, or even having heard of it".
More than 40,000 people were arrested and 120,000 sacked or suspended in the aftermath of the failed coup. They include police, military personnel, teachers and public servants.
Last month police arrested 1,000 people and issued arrest warrants for another 3,224 in an operation across 81 provinces.
The police force also suspended more than 9,000 officers over alleged links to Mr Gulen.
Mr Erdogan's critics say he is using the coup as a pretext to crush dissent and purge opponents.