In January 1977, Petr Pospichal signed the human rights manifesto Charter 77 and became an active dissident. On May 4, 1978 he was arrested for the first time at 18 years old. He was sentenced to 11 months imprisonment. Following his release Pospichal joined the Committee for the Defence of Unjustly Prosecuted (VONS) and was subjected to further imprisonment and threats to his life.

Petr Pospichal was also one of the founders of Czechoslovak-Polish Solidarity and became friends with leading Polish dissidents. After 1989 he was close to Polish President Kwasniewski.

He worked closely with Vaclav Havel who became President of Czechoslovakia in December 1989. Pospíchal later became the first Czech (after Czech Republic was founded) Ambassador to Bulgaria. After his return to the Czech Republic he was elected Chairman of the Board for Radio and TV Broadcasting.