An Istanbul court has ordered six human rights activists - including Amnesty International's Turkey director - to be formally arrested pending a trial on charges of aiding an armed terror group.
Four other activists were released from custody on Tuesday pending the outcome of a trial.
They have been barred from travelling abroad and have to report regularly to police.
The group was detained earlier this month at a hotel on Buyukada island while attending a training workshop, adding to concerns over human rights in Turkey.
It is not clear which terror organisation they are accused of helping.
President Recep Tayyip Erdogan had made vague accusations, saying the group was involved in a meeting that had the "nature of a continuation" of last year's failed coup attempt.
Amnesty International said the arrests are "an appalling affront to justice and a new low in Turkey’s post-coup crackdown".
The group said all 10 are suspected of "committing crime in the name of a terrorist organisation without being a member".
The six who were remanded in custody join Amnesty International Turkey’s Chair, Taner Kiliç, behind bars.
Colm O’Gorman, Executive Director of Amnesty International Ireland, said: "Turkish prosecutors have had 12 days to establish the obvious: that these ten activists are innocent. The decision to proceed shows that truth and justice have become total strangers in Turkey.
"This is not a legitimate investigation, this is a politically motivated witch-hunt that charts a frightening future for rights in Turkey."
The group said the "bizarre accusations" include attempts to link Idil Eser with three unrelated and opposing terrorist organisations through her work for Amnesty International.
The prosecutor's request that she be remanded in pre-trial prison custody references two campaigns by Amnesty International, neither of which were authored by Amnesty Turkey, one of which was conducted before she joined the organisation, they said.
They also claimed that an accusation levelled against İlknur Üstün of the Women's Coalition, who was released on bail, is that she requested funding from “an embassy” to support a project on “gender equality, participation in policy making and reporting.”
Mr O'Gorman said: "Today we have learnt that standing up for human rights has become a crime in Turkey. This is a moment of truth, for Turkey and for the international community. Leaders around the world must stop biting their tongues and acting as if they can continue business as usual.
"They must bring pressure to bear on Turkish authorities to drop the investigation and to immediately and unconditionally release the rights defenders."