During the first week of the Warsaw Pact invasion of Czechoslovakia in August 1968, Havel assisted the resistance by providing an on-air narrative via Radio Free Czechoslovakia station (at Liberec). Following the suppression of the Prague Spring in 1968, he was banned from the theatre and became more politically active. He took a job at Krakonoš brewery in Trutnov, an experience he wrote about in his play "Audience". This play, along with two other "Vaněk" plays (so-called because of the recurring character Ferdinand Vaněk, a stand in for Havel), became distributed in samizdat form across Czechoslovakia, and greatly added to Havel's reputation of being a leading dissident (several other Czech writers later wrote their own plays featuring Vaněk). This reputation was cemented with the publication of the "Charter 77" manifesto, written partially in response to the imprisonment of members of the Czech psychedelic rock band The Plastic People of the Universe; Havel had attended their trial, which centered on the group's non-conformity in having long hair, using obscenities in their music, and their overall involvement in the Prague underground movement. Havel co-founded the Committee for the Defense of the Unjustly Prosecuted in 1979. His political activities resulted in multiple imprisonments by the authorities, and constant government surveillance and questioning by the secret police (Státní bezpečnost). His longest period in prison, from May 1979 to February 1983, is documented in letters to his wife that were later published as "Letters to Olga".