Amnesty International claimed Turkish police arrested the director of the organisation's Turkish section and some human rights activists in what it called a "grotesque abuse of power".
The group said director Idil Eser was detained on Wednesday (5 July), along with seven other human rights defenders and two trainers, during a digital security and information management workshop at a hotel in Büyükada island, near Istanbul. The hotel owner was also arrested.
Turkish daily newspaper Hürriyet said 12 people were arrested.
Amnesty said the whereabouts of those arrested are unknown and is calling for their immediate and unconditional release.
Authorities and the police have not yet commented on the arrests.
Salil Shetty, Amnesty International's secretary general, slammed the sudden detention as a "grotesque abuse of power" which he said "highlights the precarious situation facing human rights activists in the country".
The organisation said those detained have been denied the possibility to speak with a lawyer and their families. "These detentions come less than a month after Amnesty International's Turkey chair, Taner Kiliç, was remanded in prison custody on baseless charges," Amnesty said.
Kiliç was arrested along with 22 lawyers in June. He was charged with membership of a terrorist organisation after being accused of using an encrypted messaging application called Bylock, which the government says is used by followers of the US-based cleric Fethullah Gulen. Kiliç denied the allegations.
Following the failed attempt to overthrow the Erdogan government, Turkey has cracked down on people suspected of being affiliated with Gulen and his religious movement, Hizmet.
The country has been accused of curbing freedom of speech and curtailing other freedoms after sacking thousands of civil servants, academics, police and army officials following. Following an April referendum that granted President Erdogan sweeping new powers, the country has suspended at least 14,000 civil servants, police and military personnel.
Amnesty International has openly criticised the crackdown and is calling for the release Kiliç, whose detention, the group says, is "proof of just how widespread and arbitrary Turkey's post-coup crackdown has become".