Turkish police on Friday have detained 18 academics on charges of "explicitly insulting the Turkish nation, Turkish Republic, the parliament (TBMM), Turkish government, and the judicial organs" as well as "conducting terror propaganda."

The Kocaeli Chief Public Prosecutor's Office launched an investigation into 21 academics working at the University of Kocaeli (KOU), and issued a detention warrant for them. 15 of them have been so far detained. Operations to detain the remaining suspects are reportedly in progress.

Another three academic from Uludağ University in the neighboring province of Bursa were also detained for questioning on Friday.

The academics are accused of singing a declaration describing security operations against PKK terrorists in the country's southeast as "massacre."

The manifesto was signed by about 1,128 academics working in various universities across the country. The figures call themselves as the 'Group of Academics for Peace.'

Their statement called for an end to the security forces' ongoing counter-terrorism operations amid curfews, accusing the state of violating human rights and of conducting a "deliberate and planned massacre."

Foreign Minister Mevlüt Çavuşoğlu on Friday criticized the academics' declaration, saying that he does not approve of their solidarity with the PKK organization. "Where in the world does supporting a terrorist organization and being in solidarity with it can be an expression of freedom of expression?" he asked.

President Recep Tayyip Erdoğan has also touched upon this issue on Friday, saying: "Those standing by the perpetrators of the massacres are a party to the crime." The president made the comments after attending Friday prayers at Sultanahmet mosque and leaving carnations at the scene of Istanbul suicide bombing.

"Our people must understand who is who -- having a PhD title doesn't necessarily make you an intellectual. These are people in the dark. They are cruel and despicable," Erdoğan said.

Formed in 1978, the PKK has been fighting Ankara for an independent state until the early 2000s. The group then shifted its goal to autonomy in predominately Kurdish inhabited regions of Turkey. The PKK, which the U.S. and EU also consider a terrorist organization, resumed its 30-year armed campaign against the Turkish state in late July following a two-year ceasefire.