Taraneh Alidoosti (File Photo: AFP)
"In solidarity with the peaceful struggle she is carrying out for freedom and women's rights, the @Festival_Cannes extends their full support to her," the festival wrote on Twitter.
Alidoosti, 38, was arrested on Saturday, official media said, after making a string of social media posts supporting the protest movement -- including removing her headscarf and condemning the execution of protesters.
The unrest was sparked by the September 16 death in custody of Mahsa Amini, 22, whom the morality police accused of violating the Islamic Republic's strict dress code for women.
Alidoosti, who starred in the Oscar-winning "The Salesman", was in Cannes last May to promote "Leila's Brothers".
The festival tweeted that it "strongly condemns this arrest and demands her immediate release."
A spokesperson of the Office of the UN High Commissioner for Human Rights also said they were "seriously concerned" by the detention of Alidoosti "for simply expressing her views on the current protests and particularly the execution of Mohsen Shekari, the first protester to be executed in the context of demonstrations."
British pop group, the Pet Shop Boys, also tweeted about the case, saying: "Iran's fascist government has executed protestors recently which Taraneh has condemned."