ANKARA (Reuters) - Lawyers for Turkish President Tayyip Erdogan filed a lawsuit against the main opposition leader on Monday for implying that Erdogan was a dictator, presidential sources and the opposition party said.

Erdogan, Turkey's most popular politician, is known for his intolerance of criticism and his readiness to take legal action over perceived slurs. Last week he urged prosecutors to investigate scores of academics for signing a declaration criticizing military action in the mainly Kurdish southeast of Turkey.

"Academics who express their opinions have been detained one by one on instructions given by a so-called dictator," Republican People's Party (CHP) leader Kemal Kilicdaroglu said on Saturday, referring to the petition's signatories, who oppose the military crackdown on the outlawed Kurdistan Workers' Party (PKK) and have urged an end to curfews.

"You may not agree with the content of the declaration. We also have issues with it, we also have our disagreements. But why limit freedom of speech?" Kilicdaroglu said in comments to his party's congress in Ankara.

A prosecutor from the Ankara prosecutors' office also launched an investigation into Kilicdaroglu's comments on charges of "openly insulting the president", local media reports said. The prosecutor was not immediately available for comment.

In Turkey, insulting the president is a crime punishable by up to four years in jail. As a lawmaker Kilicdaroglu enjoys immunity from prosecution, though parliament could vote by a simple majority to remove that protection.

Last Friday Turkish security forces briefly detained 27 academics accused of terrorist propaganda. Erdogan denounced the more than 1,000 signatories of the petition, who include U.S. philosopher Noam Chomsky, as "dark, nefarious and brutal".

Erdogan, who has dominated Turkish politics for more than a decade, first as prime minister and since 2014 as its first popularly elected president, has sued a number of people in the past, including cartoonists, teenagers and a former Miss Turkey winner.

(Reporting by Ece Toksabay, editing by Jonny Hogg and Gareth Jones)