According to activists, Maedeh Hojabri was one of a number of users behind popular Instagram accounts who have been arrested. The identities of the other detainees have not been confirmed.
Her account, which has been suspended, was reported to have had more than 600,000 followers.
Hojabri has since appeared on a state television programme with other detainees, in which she and others made what activists say were forced confessions, a tactic often used by Iranian authorities.
State TV showed a young woman, her face blurred, crying and shaking while describing her motivation for producing the videos.
“It wasn’t for attracting attention,” she said. “I had some followers and these videos were for them. I did not have any intention to encourage others doing the same … I didn’t work with a team, I received no training. I only do gymnastics.”
Little is known about Hojabri’s personal life, or which city in Iran she is from, but since her arrest her videos have been shared by hundreds of people, giving her a reach beyond her account.
They appear to have often been taken using a camera in her bedroom while she danced to western pop and rap music without wearing a hijab, which is required in public.
In another video, she talked about the history of parkour, an outdoor sport popular in Iran, especially among women who practise it while wearing the headscarf.
Hossein Ronaghi, a blogger, said: “People would laugh at you if you tell anyone in the world that [in Iran] they arrest 17-year-olds and 18-year-olds for dancing, being happy and being beautiful, for spreading indecency, and instead paedophiles are free.”
Many people think authorities will ban Instagram, which remains one of the few unblocked western apps. Facebook and Twitter are filtered.
The head of Tehran’s cyber-police, Touraj Kazemi, said his forces were identifying and would take action against popular accounts on Instagram. In 2012, Iran sacked the head of its cyber-police after the blogger Sattar Beheshti died in custody.
In 2014, a group of Pharrell Williams fans arrested for filming themselves dancing to the song Happy on the rooftops of Tehran received suspended sentences of imprisonment and lashes.
