Film directors Pedro Almodóvar, Ken Loach, Béla Tarr and Wim Wenders have co-signed a letter to Russian authorities concerning the detention of Ukrainian film-maker, Oleg Sentsov.
Sentsov was arrested in May at his home in Simferopol, Crimea, and charged with attempting to organise a terrorist attack.
Russia's Federal Security Service has declared that the director has admitted to plotting attacks on railway bridges, power lines and public monuments.
His lawyer - who has also campaigned on behalf of Russian punk band Pussy Riot - says that Sentsov denies any involvement. Similarly, a German film fund who supported his movies insist that Sentsov was merely a protester against the annexation of Crimea by Russia.
The European Film Board is writing to Vladimir Putin and others at the Kremlin, requesting that the safety of Oleg Sentsov be ensured. They want his place of detention to be made public. They require that the detained be charged with a recognisable offence or released.
They also demand "a full, prompt and impartial investigation into the apparently arbitrary detention by the FSB in order to bring all those responsible to justice."
Directors Stephen Daldry and Mike Leigh along with producers Mike Downey and Rebecca O'Brien have also lent their support.
Sentsov's debut feature, Gaamer, about a video game tournament, was acclaimed on the eastern European festival circuit, but work on his most recent film has been postponed.