He was a founding member of the Lithuanian Helsinki Group in 1976 which set out to document violations of human rights in the Soviet Union.

As a high school student in Raseiniai, he was an active member of Ateitis, a Lithuanian Catholic youth organization.

[2] He returned to Vilnius in December 1965 and took various short-lasting jobs (technician of medical equipment, accountant, sacristan at the Church of Saint Nicholas, etc.).

The committee published documents related to the Soviet occupation of the Baltic states and attempted to collaborate with Russian and Ukrainian dissidents, but disintegrated after arrests of its leaders.

Soviet officials searched his apartment and found two typewriters, an underground press and, documents from the Lithuanian Helsinki Group.

[2] For example, his case was mentioned by U.S. President Jimmy Carter in a news conference when discussing human rights in the Soviet Union.

[8] Harsh sentences handed out to Petkus, Alexander Ginzburg, Aleksandr Solzhenitsyn, and other dissidents prompted the U.S. government to enact punitive measures against the Soviet Union, including requiring export licenses for oil and gas equipment and reviewing cultural and scientific exchanges.

[9] When Soviets introduced glasnost and perestroika policies that allowed freer political expression, Petkus was released.

He also published two volumes of documents of the Lithuanian Helsinki Group (1999 and 2007) and a collection of letters, memoirs, and poems of political prisoners of the Soviet Union (2007).