Celal Celik, a lawyer for the secular main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu, was taken into custody in Ankara after a police raid at his home, state-run news agency Anadolu said.
According to his biography on the party's official website, Celik is a former judge who now also works as a member of the party's High Disciplinary Committee (YDK).
Celik is accused of being close to a former science, technology and communications advisor to Kilicdaroglu, Fatih Gursul, who was detained in December over alleged links to the Muslim cleric Fethullah Gulen, Anadolu reported.
The detention comes after allegations Celik cancelled his subscription with satellite TV provider Digiturk after it removed Gulen-linked television channels Bugun and Samanyolu, Anadolu and private news agency Dogan said.
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There were suspicions he cancelled it because of "instructions from Gulen", Dogan news agency reported.
Celik will be taken to Istanbul, Anadolu said, as the investigation is conducted by the Istanbul chief public prosecutor.
Over 50,000 people have been arrested since July 2016 over suspected connections with Gulen while nearly three times that have been suspended or fired from the public sector.
Critics including Kilicdaroglu say the purge has gone well beyond the coup plotters and targeted all kinds of dissidents but the government argues it is necessary to remove real threats to the state.
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