A Turkish prosecutor ordered the detention of the head of the Turkish Medical Association (TTB) and 10 other leaders of the union on Tuesday, after it criticized Ankara’s military operation against the Kurdish People's Protection Units in northern Syria.
The prosecutor said police in Ankara started legal proceedings on Tuesday morning and search-and-detention operations were going on in eight provinces.
TTB head Rasit Tukel and other members of the board had been detained, opposition CHP lawmaker Ali Seker said on Twitter.
The association's lawyer Ziynet Ozcelik said they faced accusations of "propaganda in support of a terrorist organization, and provoking the public".
Ozcelik said it was the first time in the association's history that all its executive members had been ordered detained.
The Turkish Medical Association denounced the cross-border operation into Syria's Afrin last week, saying "No to war, peace immediately".
Ankara refers to the YPG as a "terrorist" offshoot of the outlawed Kurdistan Workers' Party (PKK), waging an insurgency against the Turkish state since 1984.
The PKK is blacklisted as a terror group by Ankara and its Western allies.
President Recep Tayyip Erdogan accused the TTB of treason on Sunday.
"Believe me, they are not intellectuals at all, they are a gang of slaves. They are the servants of imperialism," he told AK Party members in the northern province of Amasya.
"This 'No to war' cry by this mob ... is nothing other than the outburst of the betrayal in their souls ... This is real filth, this is the honorless stance that should be said 'no' to," Erdogan said.
The interior ministry on Monday said 311 people including journalists and activists had been detained over accusations they were spreading "terror propaganda".