Idil Eser, director of Amnesty International Turkey, was held on Wednesday along with activists and trainers on Buyukada, an island south of Istanbul.
Amnesty International said those arrested are accused of membership of a terror group.
Erdogan, when asked about the activists at a news conference at the closing session of G20 in Hamburg, referred back to the attempt to topple him.
"They gathered for a meeting which was a continuation of July 15," he said, referring to the date when his rule was threatened, noting that any action regarding the activists rests with the judiciary.
Also Read
The detentions come less than a month after Amnesty's Turkey chair, Taner Kilic, was remanded in custody on what the group described as "baseless charges" of links to Fethullah Gulen, a US-based cleric and alleged mastermind of the coup.
Turkey remains under a state of emergency, which critics say has been used to round up not just the alleged plotters but anyone who dares to oppose the president.
Disclaimer: No Business Standard Journalist was involved in creation of this content