Turkish police arrest Syria critics under 'terrorist propaganda' charges
Among those arrested was journalist Hakan Demir, the head of Birgun, an opposition news outlet.
He was detained for "inciting the people to hatred and enmity" after it reported there had been civilian casualties in the offensive, which was launched by Turkish and Syrian forces on Wednesday.
Ankara has denied civilian casualties in the operation, dubbed "Operation Spring of Peace", which began on Wednesday, which aimed at capturing parts of northern Syria from the Kurdish-led Syrian Democratic Forces (SDF).
Demir was later released but had his passport confiscated.
Police also arrested 21 people in the Kurdish-majority city of Mardin in southeastern Turkey for "terrorist propaganda", according to state news agency Anadolu.
They had already announced that 78 cases had been opened against individuals nationwide.
Meanwhile, Ankara's chief prosecutor said cases had also been opened against the leaders of the pro-Kurdish Peoples' Democratic Party (HDP), Sezai Temelli and Pervin Buldan, and other members of the party.
They had described the military operation as "an invasion" in a statement, and Temelli maintained his position on Thursday, saying it was an act of "aggression" and "attempted occupation".
Criticism of military operations in Turkey is fiercely taboo, with even opposition parties required to praise them.
Ankara arrested online critics during previous offensives against Kurdish forces in Syria, accusing them of "terrorist propaganda".
Rights groups criticise the erosion of free speech under President Recep Tayyip Erdogan, particularly in the wake of a failed coup in 2016 which was followed by tens of thousands of arrests of political opponents.