A video from Iran showing a woman being violently attacked for her "insufficient" head covering by what activists say are the country's so-called morality police went viral on Thursday.

In the video, the woman shrieks and weeps as she is manhandled and thrown to the ground by women whose black robes expose only their faces. A crowd gathers around as the attack continues, and at one point a woman who does not appear to be part of the religious police is seen attempting to comfort the victim.

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The video was shared on Twitter by Masih Alinejad, who hosts the website and Facebook group My Stealthy Freedom, where women post photos of themselves without hijabs in Iran.

"This woman is savagely beaten up by morality police as punishment for her insufficient hijab. And they tell us hijab is a 'small issue,'" Alinejad wrote in her tweet. Some Twitter users wrote that Iranian officials are looking into the violent incident and have ordered an investigation.

Under Iran's Islamic law, imposed after the 1979 revolution, women are obliged to cover their hair with a scarf, known as a hijab, and wear long, loose-fitting clothes. Violators are publicly admonished, fined or arrested.

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To campaign against the obligatory wearing of headscarves, Alinejad last year encouraged women to take videos or photos of themselves wearing white and upload them on social media with the hashtag #whitewednesdays. Last February, 29 women who took part in the campaign were arrested.

The #whitewednesdays campaign is part of a larger online movement started three years ago by Alinejad, a journalist who has lived in self-imposed exile since 2009. She has received death threats since her campaigning started.