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 reported.
Kilicdaroglu told reporters the detention was "a disgrace in the name of democracy", adding the situation was "an abdication of reason".
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.
Turkey accuses Gulen, who lives in self-imposed exile in Pennsylvania, of ordering the attempted putsch -- allegations which he strongly denies.
The detention comes after accusations 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.
There were suspicions he cancelled it because of "instructions from Gulen", Dogan reported.
'Fought against Gulen'
Kilicdaroglu pointed to Celik's resignation from the Supreme Court of Appeals in 2011 as evidence of his opposition to Gulen.
Gulen was once a close ally of President Recep Tayyip Erdogan but the two fell out in recent years as Erdogan became suspicious of Gulen's movement -- and its powerful presence in Turkish society -- after 2013.
"During the days when (the Gulen movement) was dominant in the Supreme Court of Appeals, he was a lawyer forced to resign... and resigned to rebel against (this)," he said.
CHP spokesman Bulent Tezcan told reporters Celik was "a man who fought against" the Gulen movement throughout his working life.
"It's not possible to accept this," Tezcan said, as he vowed Celik had no Gulen links.
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.
Kilicdaroglu has been fiercely critical of the government and the subsequent crackdown, accusing Erdogan of a "civilian coup".
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.
The Ankara chief prosecutor issued another 71 arrest warrants for people accused of links to Gulen, including eight individuals who work in the education ministry, Anadolu said. At least 27 had already been detained, it added.
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