Tarto began his career as an activist for democracy, human rights and national independence already as a youth. He was sentenced to prison as a minor in 1957 together with seven of his high school classmates in the city of Tartu who distributed homemade leaflets condemning the Soviet invasion of Hungary in 1956.
He was released from the gulag in 1960, only to be rearrested in 1962. After returning from the gulag to Estonia in 1967, he was allowed to study at Tartu University in 1969 but was expelled two years later. He worked as a furnace stoker, a typical job for a dissident since it kept him from workplace interaction with other citizens. In 1979 he was one of the authors of the Baltic Appeal to the United Nations which denounced the secret protocols of the 1939 Nazi-Soviet Pact and demanded the restoration of the independence of the Baltic states.
During the renewed crackdown on dissidents and human rights activists in the early 1980s, Tarto was arrested again and sentenced to a ten-year prison term in 1983. Tarto was the final Estonian political prisoner to be freed from the gulag in 1988.
Upon his return to Estonia, Tarto has been active in politics, first as an independence movement activist and then serving as a member of the national parliament.