Turkish security forces have re-arrested two activists previously detained but then released in a controversial case that has raised tensions with the West, Amnesty International said.
The two were among 10 people detained earlier this month in a raid by police on a workshop session of human rights activists held on an island off Istanbul.
A Turkish court on Tuesday ordered six of the human rights activists, including Amnesty International's Turkey director Idil Eser, to be remanded in custody on charges of aiding a "terror" group.
The four others were then released under judicial supervision.
But an Istanbul court on Friday issued new arrest warrants for the four - Nalan Erkem, Seyhmus Ozbekli, Nejat Tastan and Ilknur Ustun - after granting an appeal from prosecutors against their release.
Amnesty said Ms Erkem was detained from her house in Istanbul late Friday and Ms Ustun was detained from her home in Ankara on Saturday.
There was no immediate indication of the whereabouts of Mr Ozbekli and Mr Tastan.
John Dalhuisen, Amnesty International's Director for Europe described the new detention orders as a "cruel and retrograde step" and said the Turkish authorities have "raised their absurdity to fresh heights".
"Turkey has underlined its growing reputation as an indiscriminate jailer of civil society activists and a stranger to the rule of law," he said.
The decision to remand the six in custody earlier this week sparked international alarm and amplified fears of declining freedom of expression under president Recep Tayyip Erdogan.
Eight of the 10 initially detained are Turkish rights activists. But the other two are German Peter Steudtner and Swede Ali Gharavi, who were leading the digital information workshop.
This has stoked tensions in particular with Berlin, which is now looking at an overhaul of its relations with Ankara.
Amnesty describes Mr Gharavi as an IT strategy consultant and Mr Steudtner as a "non-violence and well-being trainer".
Last month Amnesty International's Turkey chair, Taner Kilic, was remanded in custody on what the group described as "baseless charges" of links to the alleged mastermind of the July 15 failed coup Fethullah Gulen.
