Turkish police have arrested nine leading human rights activists, including the local director of Amnesty International, in a raid near Istanbul.
Two trainers, from Germany and Sweden, were also arrested as they worked in a digital security workshop at a hotel.
No reason has been given for the arrests. The operation was a "grotesque abuse of power", Amnesty said.
Meanwhile, tens of thousands of people are marching from Ankara to Istanbul as part of an opposition protest.
The march, now on its 22nd day, was in reaction to the arrest of an opposition lawmaker. But it has become a wider demonstration of what many say is an erosion of democracy under President Recep Tayyip Erdogan.
Turkey remains under a state of emergency imposed after rogue army officers tried to oust Mr Erdogan in a coup on 15 July 2016. Police have jailed more than 50,000 people since then.
The human rights activists were arrested on Buyukada, an island south of Istanbul, and were being held in five different police stations. They included Idil Eser, Amnesty's Turkey director.
The group's secretary general, Salil Shetty, said in a statement that the raid was "a grotesque abuse of power and highlights the precarious situation facing human rights activists in the country", and urged their immediate and unconditional release.
The workshop was organised by Netherlands-based rights group Hivos.
Human rights activists are often accused of treacherous activity in Turkey but the new development is a serious escalation of the situation, the BBC's Selin Girit reports from Istanbul.
Amnesty International's Turkey chair, Taner Kilic, is also in police custody. He was arrested on 6 June with 22 other lawyers and charged with membership of a "terrorist" organisation. Amnesty called the charges "baseless".
Seven of the rights activists arrested with Ms Eser on Wednesday evening were named by Amnesty as: Ilknur Ustun of Women's Coalition; Gunal Kursun and Veli Acu of Human Rights Agenda Association; Nalan Erkem and Ozlem Dalkiran of Citizen's Assembly; Nejat Tastan of Equal Rights Watch Association and lawyer Seyhmus Ozbekli.
The post-coup crackdown has targeted tens of thousands of public servants accused of supporting US-based Muslim cleric Fethullah Gulen.
The European Parliament has deplored the crackdown on Mr Erdogan's opponents in Turkey and called for a suspension of talks on it joining the EU if Mr Erdogan is formally granted sweeping new powers.
A controversial referendum in April backed constitutional changes that would turn Turkey into a presidential republic, diminishing parliament's role.
Turkey has made very slow progress towards EU membership, amid international concern about freedom and justice under Turkey's current state of emergency.