DIYARBAKIR, Turkey (Reuters) – Turkish police launched criminal investigations of Kurdish lawmakers and detained dozens of people for using social media to question the military’s cross border incursion into Syria, in a crackdown against those who criticized the assault.

Police fired water cannon and detained dozens of activists on Thursday in the mainly Kurdish southeastern city of Diyarbakir to prevent a protest against the assault into northern Syria.

Turkey launched the assault against the Kurdish YPG militia on Wednesday after a withdrawal by U.S. forces with who had been allied with the Kurds. Ankara deems the YPG a terrorist group over its links to militants who have waged an insurgency in Turkey.

Reuters journalists were present at the protest in front of the pro-Kurdish People’s Democratic Party (HDP) headquarters in Diyarbakir, the biggest city in the southeast. Police in riot gear fired water cannon and then detained protesters. A party official said police detained 25 party members in the city.

The protest was initially called against the ousting of HDP mayors but was also to show opposition to the Syria operation, a party official said. The HDP’S Diyarbakir provincial leader and other party officials were among those detained.

Earlier on Thursday, authorities began investigating HDP leaders and detained 21 people for criticizing the military offensive online, state-owned Anadolu news agency said.

While most of Turkey’s opposition parties have voiced support for the operation, the HDP has called for the offensive to stop, describing it as an “invasion attempt”.

HDP co-leader Sezai Temelli said on Wednesday the operation was an attempt by the government to drum up support amid declining public backing. Prosecutors launched an investigation against him and HDP’s other co-leader, Pervin Buldan, over their remarks about the offensive, Anadolu said.

It said Temelli and Buldan were accused of “carrying out propaganda for a terrorist organization” and “openly insulting Turkey’s government”. Three other HDP lawmakers were being investigated over similar charges, the news agency said.

The government accuses the HDP of ties to the Kurdistan Workers Party (PKK) militant group and thousands of its members have been prosecuted for the same reason, including its leaders. The HDP denies such links.

Hours after the operation started on Wednesday, authorities also launched investigations against 78 individuals who criticized the offensive on social media, Anadolu said.

On Thursday, 21 people were detained in the southern province of Mardin for their social media posts. The suspects are accused of “provoking the public to hatred and animosity” and “carrying out propaganda for a terrorist organization”, Anadolu said.

Authorities launched similar investigations after each of Turkey’s two previous cross-border operations into Syria. More than 300 people were detained for social media posts criticizing Turkey’s offensive into northern Syria in January 2018.

Reporting by Tuvan Gumrukcu and Daren Butler; Editing by Angus MacSwan