Idil Eser, director of Amnesty International Turkey, was detained on July 5 along with seven other activists and two foreign trainers during a digital security and information management workshop on Buyukada, an island south of Istanbul.
Their detention caused international alarm and amplified fears of declining freedom of expression under President Recep Tayyip Erdogan.
The police detention will now last until July 19, Amnesty's Turkey researcher Andrew Gardner told AFP.
Eight of those detained are Turkish human rights defenders, including Ilknur Ustun of the Women's Coalition and Veli Acu of the Human Rights Agenda Association.
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Two are foreign trainers -- a German and a Swedish national -- who were leading the digital information workshop.
They are accused of membership of an "armed terrorist organisation", an allegation Amnesty said was "unfounded".
But it is not clear which organisation they are accused of belonging to, Gardner said.
The extension is allowed under the state of emergency imposed after last year's failed coup.
Gardner argued the first part of their detention was illegal because they were denied access to lawyers for 24 hours, could not contact family members where they were and authorities refused to give their location.
"For them to be entering a second week in police cells is a shocking indictment of the ruthless treatment of those who attempt to stand up for human rights in Turkey," Amnesty's Europe director John Dalhuisen said.
"They gathered for a meeting which was a continuation of July 15," he said on Saturday.
Last month, Amnesty International's Turkey chair Taner Kilic was arrested, accused of links to US-based Fethullah Gulen, who is accused of ordering last year's failed coup.
Amnesty also dismissed those charges as "baseless".