Turkish police detained 12 people, including two prominent academics, on Friday as part of an investigation into leading rights activist and businessman Osman Kavala and his cultural organisation, state media said.
Pro-government media said they were arrested over the mass protests against Tayyip Erdogan's government which erupted at Istanbul's Gezi Park in May 2013 and spread across Turkey, in what was a major challenge to the now president's rule.
Kavala was jailed pending trial a year ago, accused of seeking to overthrow the government as part of an investigation into a network which Ankara accuses of carrying out a 2016 failed coup.
Human rights groups and European Parliament members have repeatedly called for his release and an indictment against him has not yet been issued.
The two academics were named as Turgut Tarhanli, dean of the law faculty at Istanbul Bilgi University, and maths professor Betul Tanbay of Bogazici University, who was elected this year as vice president of the European Mathematical Society.
State-owned Anadolu news agency said Istanbul prosecutors had sought the arrests of 20 suspects in total and police had carried out simultaneous raids at various addresses.
Among the detainees were staff of Kavala's Anadolu Kultur organisation, which is involved in promoting culture and rights, including board members Yigit Ekmekci and Hakan Altinay. It was not immediately clear what they were accused of.
Turkish Bar Association Chairman Metin Feyzioglu said in a written statement he was monitoring developments with concern.
"The violation of basic rights and freedoms by the hand of the judiciary must not be allowed," he said. "All illegal practices which will harm Turkey in the international community must be brought to an end."
'Stirring chaos'
Pro-government Sabah newspaper said the 20 were held over their alleged roles in the Gezi Park protests, in which 10 people were killed, two of them police officers. The government's handling of the protests drew criticism from the European Union and United States.
Kavala himself was accused of financing and organising those protests through Anadolu Kultur to "stir chaos across the country" and "foment an armed rebellion against the government", Sabah said.
In a statement two weeks ago, Amnesty International and Human Rights Watch renewed calls at the end of October for Kavala's immediate and unconditional release.
Since the attempted putsch in July 2016, Turkey has jailed 77,000 people as they face trial, as well as suspending or dismissing 150,000 civil servants and military personnel and shutting down dozens of media outlets.
Police still frequently carry out operations targeting the network of U.S.-based cleric Fethullah Gulen, who Ankara accuses of orchestrating the July 2016 failed putsch.
Anadolu said on Friday that prosecutors issued an order for the arrest of 188 people, including 100 former air force personnel, over links to Gulen's network. So far 86 suspects have been detained in the operation.