The Iranian director, a vocal supporter of the country's opposition leader Mirhossein Mousavi, was arrested following a police raid on his home in Tehran in March and has since allegedly been held in the city's Evin prison.
A group of Hollywood's most high-profile stars have now come together to fight for the filmmaker's freedom, signing a petition calling for Panahi's release.
The document reads, "We... stand in solidarity with a fellow filmmaker, condemn this detention, and strongly urge the Iranian government to release Mr. Panahi immediately. Like artists everywhere, Iran's filmmakers should be celebrated, not censored, repressed and imprisoned."
The petition has also been signed by Martin Scorsese, Francis Ford Coppola, Joel and Ethan Coen, Ang Lee, Oliver Stone and documentary maker Michael Moore.
The director, whose movies include The White Balloon and The Circle, backed Mousavi last year during the country's controversial election which saw President Mahmoud Ahmadinejad retain power.
He was briefly detained in the summer of 2009 after attending a memorial for a slain protester, and was allegedly banned from traveling abroad after he was photographed wearing green - the color of opposition supporters - at the Montreal film festival in Canada.