Per Wikipedia:
David Mallet’s music video for the song features scenes from Fritz Lang’s 1927 sci-fi movie Metropolis—Freddie Mercury’s solo song “Love Kills” was used in Giorgio Moroder’s restored version of the film, and in exchange Queen were granted the rights to use footage from it in their “Radio Ga Ga” video. However,.....
http://en.wikipedia.org/wiki/Radio_Ga_Ga