ISTANBUL, Jan. 22 (Xinhua) -- At least nine people were detained in Istanbul on Monday for promoting terror organizations on social media and opposing Turkey's ongoing military operation against Syria's Afrin, local media reported.

According to CNNTurk, police also seized a large number of organizational documents, books, magazines and digital materials in the operations.

The Istanbul Anadolu Chief Public Prosecutor's Office said in a statement that an investigation has been launched into a total of 57 people over insulting the president, the Turkish state and military, inciting the public to hatred and hostility, and promoting terror groups in tweets against the Afrin operation.

Turkey started on Saturday a military offensive against Kurdish militia in Afrin, seen by Ankara as terrorists associated with the outlawed Kurdistan Workers' Party.