Seven persons were taken to FSB directorate after the mass searches in their houses in the Russia-occupied Crimea on Wednesday.
The searches were conducted at the homes of Lenur Seydametov (Bakhchisarai), Timur Yalkabov (Akmescit), Azamat Eyupov (Qarasuvbazar), Yashar Shikhametov (Baydar village) ve Abdulboriy Makhamadaminov (Icki district), Ernest Ibragimov (Bakhchisarai) and Oleg Fedorov (Bakhchisarai).
Ukrainian Ombudswoman Lyudmyla Denisova said in a statement that all of the men were taken to the FSB's directorate in Crimea after the searches.
Abdulboriy Makhamadaminov was released shortly after.
"Such actions by so-called law enforcement violated the presumption of innocence, and rights of freedom, personal security, legal assistance, as the searches were conducted without court decisions, with no lawyers present, and without handing out of any procedural documents," Denisova said.
She added that the detained Crimean Tatars might face long-term arrests.
After the occupation of Crimea by Russia, the invaders started persecuting independent journalists, civil activists, members of the Crimean Tatar national movement, the Mejlis, as well as Crimean Muslims suspected of links with the international Islamic political organization Hizb-ut Tahrir.
Hizb-ut Tahrir is a global organization based in London that seeks to unite all Muslim countries into an Islamic caliphate, while rejecting terror tools to achieve their goal. It is banned in Russia as a terrorist organization but operates legally in Ukraine.
Rights groups and Western governments have denounced what they describe as a campaign of repression by the Russian-imposed authorities in Crimea, who target members of the Turkic-speaking Crimean Tatar community and others who have spoken out against Moscow's takeover of the peninsula.
Russia took control of Crimea from Ukraine in March 2014 after sending in troops, seizing key facilities, and staging a referendum dismissed as illegal by at least 100 countries.
The West did not recognise the occupation in response to which sanctions against Russia were introduced. Crimea remains internationally recognised territory of Ukraine.