In Poland, the pro-Soviet authorities following the Soviet occupation after the war covered up the matter in accordance with the official Soviet propaganda line, deliberately censoring any sources that might provide information about the crime. Katyn was a forbidden topic in postwar Poland. Censorship in the Polish People's Republic was a massive undertaking and Katyn was specifically mentioned in the "Black Book of Censorship" used by the authorities to control the media and academia. Not only did government censorship suppress all references to it, but even mentioning the atrocity was dangerous. In the late 1970s, democracy groups like the Workers' Defence Committee and the Flying University defied the censorship and discussed the massacre, in the face of arrests, beatings, detentions, and ostracism. In 1981, Polish trade union Solidarity erected a memorial with the simple inscription "Katyn, 1940". It was confiscated by the police and replaced with an official monument with the inscription: "To the Polish soldiers—victims of Hitlerite fascism—reposing in the soil of Katyn". Nevertheless, every year on the day of Zaduszki, similar memorial crosses were erected at Powązki Cemetery and numerous other places in Poland, only to be dismantled by the police. Katyn remained a political taboo in the Polish People's Republic until the fall of the Eastern Bloc in 1989.