The operations were launched after an offshoot of the Kurdistan Workers Party (PKK) on Sunday claimed responsibility for twin bombings that killed 38 people and wounded 155 outside an Istanbul soccer stadium.
Around dawn, about 500 police, backed by armoured vehicles and a helicopter, launched an operation in the southern city of Adana and detained 25 HDP officials, state-run Anadolu agency said.
It said counter-terror police teams in Istanbul separately took into custody 20 HDP officials, including its provincial head, and carried out searches at various addresses including the partys main offices in the city.
The leaders of the HDP, the second-largest opposition grouping in parliament, have already been jailed pending trial over alleged ties to the PKK, and Ankara regularly accuses the HDP of being an extension of the militant group.
The PKK is designated a terrorist organisation by Turkey, the United States and the European Union, Reuters reported.