Ms Bahraini said Ms Movahed was only just released on Sunday after a month in custody and that Amnesty is monitoring the case because she is still at risk of criminal prosecution.
According to reports, the mother-of-one was arrested shortly after her protest on December 27, and taken to a detention centre with her 20-month-old daughter.
Movahedi has now been freed, according to a January 28 Facebook post by leading Iranian human rights lawyer Nasrin Sotoudeh, who said Movahedi was initially released and rearrested.
The movement to get rid of the country's dress codes was started by Iranian activist Masih Alinejad and her website My Stealthy Freedom, which encouraged women to send photos of themselves without hijabs.
Iranian Islamic law has forced women to wear a hijab and long, loose clothing for modesty since the 1979 revolution.
Ms Esfandiari said the protests tie in with wider, anti-regime demonstrations that took place across 80 Iranian cities in December, when more than 1,000 people were arrested and 25 killed.
Women who appear in public without the obligatory hijab can face fines and prison sentences ranging from 10 days to two months. The announcement signaled an easing, as called for by the young and reform-minded Iranians who helped re-elect President Hassan Rouhani, a relative moderate, earlier this year. A growing number of women, predominantly in Tehran, have begun refusing to wear a hijab while driving, arguing that a vehicle is a private space where they can dress more freely.
In the photos, the women's heads are uncovered as they stand on utility boxes or other objects on the streets of Tehran with scarves held up on sticks.
The so-called "hijab protest" seems to have struck a chord with many - especially younger - Iranians.
"The fear factor is going away for some people inside the country and they are so frustrated they are not afraid anymore to take to the streets and publicly protest", she told the ABC.
"But as a woman who grew up in Iran I can tell you it's a big issue".
The Islamic code also forbids women touching, dancing or singing with men outside their families.