Simplify the following text for a Grade 9 native English-speaking person.
Original: In 1977 , his involvement with the human rights manifesto Charter 77 brought him international fame as the leader of the opposition in Czechoslovakia ; it also led to his imprisonment .
Simplified: In 1977 , he became famous internationally for his work on the human rights manifesto , Charter 77 . He became known as a leader of the opposition in Czechoslovakia .