The actresses were reportedly jailed after a prosecutor launched an investigation into their social media activities.
On Saturday, Hengameh Ghaziani hinted that she had been subpoenaed by law enforcement and posted a video on Instagram of her removing her veil.
"Perhaps this will be my last post," she wrote, adding that no matter what happens, she will "be with the Iranian people until my last breath."
Ghaziani, who has received several awards, has been a vocal critic of the Iranian regime's crackdown on protesters and has, among other things, accused the regime of having "murdered more than 50 children".
According to the state news agency Irna, she must have been arrested for having "incited" and supported the protests, called "riots" by the authorities.
Riahi has expressed his solidarity
Katayoun Riahi, who has appeared in a number of award-winning films and is also known for her charity work, appeared in an interview with London-based Iran International TV in September, where she did not wear a veil.
Riahi has expressed her solidarity with the protests as well as opposition to the mandatory hijab law.
Iran has been rocked by protests since September 16, when 22-year-old Mahsa Zhina Amini died in the custody of hardline morality police, where she was taken for violating the country's strict dress code for women.
Recently, the famous Iranian actress Taraneh Alidoosti also expressed her support for the protests against the regime in Iran, by posting a picture without a veil on social media.