Idil Eser, the director of UK-based Amnesty International's Turkey branch, was among the 10 people detained on Wednesday while holding a workshop at a hotel on Buyukada, an island in Istanbul.
Amnesty said that, in addition to Eser, seven other Turkish activists, two trainers - German and Swedish nationals, and the hotel owner were detained.
"Unfortunately, they gathered there for a meeting which has the nature of a continuation of July 15," Erdogan said on Sunday, referring to the date of the coup attempt last year.
"Police forces raided [the hotel] based on information the intelligence [agency] received. And as a result of this raid, they were detained," he said in a televised news conference during the G20 summit in Hamburg.
Erdogan added that the rest of the process is up to the judiciary, which is expected to decide if the detainees will be charged or not.
Andrew Gardner, researcher for Amnesty International on Turkey, said that all the activists in custody were being questioned over was "being members of an armed terrorist organisation".
"This is a ludicrous accusation with no base. The workshop is a perfectly simple routine professional workshop, which human rights defenders would have all over the world. There is nothing suspicious, secret or illegal about it," he told Al Jazeera.
Gardner said that the locations of the detainees were unknown for 28 hours after they were taken into custody, adding that they were not allowed to talk to any relatives and had no access to their lawyers during that time.
"What we hope and expect is, since there is absolutely no base for these detentions, for them to be released without further delay," Gardner said.
The detentions have been authorised for seven days, Amnesty said, adding that they can be extended for a further seven days without the activists being brought before a court.
Taner Kilic, the chairman of Amnesty International Turkey, was also arrested in early June for having alleged links to US-based religious leader Fethullah Gulen, who Turkey accuses of masterminding last year's failed coup attempt. He is waiting for an indictment and to appear in court, potentially facing prison time.
Around 300 people were killed during the coup attempt and it led to arrests as well as purges targeting tens of thousands of civil servants.
The government, which declared a state of emergency, say the purges and detentions are legitimate, aiming at removing Gulen supporters from state institutions and other parts of society.
Local and international rights groups, as well as many of Turkey's European allies, say the measures are arbitrary, claiming that the government is using the coup attempt as a pretext to silence opposition in the country.
Umut Uras is on Twitter, follow him on @
Source: Al Jazeera News