In a rare rebuke to Washington's NATO ally, the US ambassador to Turkey expressed concern over the effect of the investigations against the academics on a healthy political debate over the violence in the southeast.

Western concern over freedom of expression in Turkey is already running high due to the detention since November 26 of two prominent journalists of the opposition daily Cumhuriyet.

Police had early Friday launched an operation to detain 21 Turkish academics as part of an investigation accusing them of disseminating "terror propaganda" by signing a petition denouncing military operations against Kurdish rebels.

Police have so far detained 14 academics and lecturers from the University of Kocaeli, near Istanbul, in an early morning raid on their homes, Dogan news agency said.

Operations to detain the remaining seven suspects are in progress, it added. Police also searched the homes of three academics in northwestern Bolu province, Dogan said.

Prosecutors on Thursday launched a vast investigation into over 1,200 academics from 90 Turkish universities for engaging in "terrorist propaganda" and "inciting hatred and enmity" by signing the petition.

Entitled "We won't be a party to this crime", the petition urged Ankara to halt "its deliberate massacres and deportation of Kurdish and other peoples in the region."

Turkey is waging an all-out offensive against the Kurdistan Workers' Party (PKK), with military operations backed by curfews aimed at flushing out rebels from several southeastern urban centres.

But Kurdish activists say dozens of civilians have died as a result of excessive force and the operations have become the subject of huge controversy in Turkish society.

- 'Plunging Turkey into darkness' -

Erdogan on Friday launched his strongest attack yet on the signatories, accusing them of supporting the Kurdish rebels and thus being a "party" to the crimes of the PKK.

"Those standing by the perpetrators of the massacres are a party to the crime," he told reporters after Friday prayers in Istanbul.

"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."

Critics have denounced the move as the latest attempt by Erdogan to stifle dissent, with pro-Kurdish People's Democratic Party (HDP) condemning "the steps taken by the Turkish government that are plunging Turkey into a deep darkness."

"These operations... which are only be seen in undemocratic regimes are very dangerous and unacceptable," the main opposition Republican People's Party (CHP) added in a statement.

US ambassador to Ankara John Bass expressed concern over the "pressure having a chilling effect on legitimate political discourse across Turkish society regarding the sources and solutions to the ongoing violence."

"In democratic societies it is imperative that citizens have the opportunity to express their view, even controversial or unpopular ones," he added in a statement.

"Expressions of concern about violence do not equal support for terrorism. Criticism of the government does not equal treason," he added.

University authorities on Friday have also opened probes into 20 academics in Mersin University on the Mediterranean and four others in southeastern city of Gaziantep that could potentially lead to their dismissal.

Earlier on Thursday, Duzce University in northwest Turkey dismissed an associate sociology professor after she signed the declaration, Turkish media reports said.

The petition was also signed by dozens of foreign luminaries and intellectuals, among them American linguist Noam Chomsky and the Slovenian philosopher Slavoj Zizek.

Erdogan has publicly criticised Chomsky but so far none of the foreign signatories have been put under investigation in Turkey.

The United States and EU have already expressed concern over the imprisonment pending trial of Cumhuriyet daily editor Can Dundar and the paper's Ankara bureau chief Erdem Gul for publishing articles alleging the government delivered weapons to Islamists in Syria.