Turkish police on Friday detained 21 academics for "terror propaganda" over a petition denouncing military operations against Kurdish rebels in the southeast that was severely criticised by President Recep Tayyip Erdogan, the state media said.

Police detained the academics from the University of Kocaeli, near Istanbul, in an early morning raid on their homes, the official Anatolia news agency said.  

The move came after prosecutors launched a vast investigation into over 1,200 academics from 90 Turkish universities for "insulting the state" and engaging in "terrorist propaganda" by signing the declaration. 

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." 

Erdogan on Thursday accused the signatories, which also included American linguist Noam Chomsky and the Slovenian philosopher Slavoj Zizek, of "treason" and being the "fifth columns" of foreign powers bent on undermining Turkey's national security.

"You are people in the dark. You are not intellectuals!" he said. "All you want is to stir this country up." 

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.

Kurdish activists say dozens of civilians have died as a result of excessive force.