ISTANBUL (Reuters) - Turkish police detained 13 people including two prominent academics on Friday under an investigation into jailed rights activist Osman Kavala, accusing them of following him in a bid to unseat the government through mass protests in 2013.

Kavala, a well-known civil society leader, was jailed a year ago pending trial for trying to overthrow the government, a process which authorities say started with those demonstrations and culminated with an attempted coup in July 2016.

Police said those arrested on Friday were accused of working with Kavala to foment the May 2013 unrest, which began as demonstrations against urban development plans at Istanbul’s Gezi Park and turned into nationwide anti-government protests.

At least 10 people died and thousands were injured in a government crackdown on the demonstrations, which at the time were the biggest ever staged against the government of Tayyip Erdogan, now president, then entering his second decade as prime minister.

Kavala was accused of attempting to use force to remove the government or prevent it from fulfilling its duties, the police said. Prosecutors ordered the arrest of 20 people in total accused of acting with him. The charge carries a life sentence.

“The individual aimed to spread the Gezi Park incidents across Turkey and create an atmosphere of chaos and disorder in the country generally,” Istanbul police said in a statement.

Human rights groups and European Parliament members have repeatedly called on NATO member Turkey to release Kavala. An indictment against him has not yet been issued.

On Friday, the European Union called for the immediate release of “all those detained without due process”.

“Repeated detentions of critical voices and the continued widespread pressure on civil society representatives run counter to the Turkish government’s declared commitment to human rights,” the EU’s foreign policy service said.

The U.S. State Department said it was “very concerned” about the detentions and urged Turkey to respect “human rights and fundamental freedoms, and to release those held arbitrarily”.

A lawyer for one of the suspects named the two academics detained on Friday as Turgut Tarhanli, dean of Istanbul Bilgi University’s law faculty, and Bogazici University’s Betul Tanbay, who was elected this year as vice president of the European Mathematical Society.

Among the detainees were also staff of Kavala’s Anadolu Kultur organization, which is involved in promoting culture and rights, including board members Yigit Ekmekci and Hakan Altinay.

Turkish Bar Association Chairman Metin Feyzioglu said 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 in a statement. “All illegal practices which will harm Turkey in the international community must be brought to an end.”

“PROFESSIONAL PROTESTERS”

Police said the suspects had brought in professional protesters from abroad to fuel the Gezi Park protests and had lobbied in Europe to halt the import of tear gas used by police.

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, accused by Ankara 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.