Mardin Metropolitan Mayor Ahmet Türk has been detained as part of the ongoing terror probe on links between the PKK terrorist organization and HDP municipalities.
Türk was suspended from duty on Thursday, and was detained along with Artuklu district Mayor Emin Irmak on Monday.
According to reports, counter-terror police detained both mayors in their houses.
Earlier this month, 13 lawmakers from the HDP were arrested. 10 of them, including co-chairs Selahattin Demirtaş and Figen Yüksekdağ, remain in custody awaiting trial on terrorism-related charges.
In October, the DBP co-mayors from Diyarbakır, the largest city in the southeast, were arrested and jailed pending trial on terrorism charges.