LA PAZ, Bolivia—Sean Penn has made a direct public appeal to Bolivia's president to order the immediate release of a New York businessman who has been jailed for 18 months without charge and who helped uncover a shakedown ring led by prosecutors.

Penn made the appeal at a news conference with Jacob Ostreicher on Wednesday, a day after an appeals panel sent the man's case back to a lower court. The actor interceded on Ostreicher's behalf in late October, and a government probe subsequently yielded eight arrests, with more expected.

Penn thanked President Evo Morales for ordering the investigation. He called the shakedown ring "a cancer attacking at Bolivia's heart" and creating an "existential crisis" for Morales.

Penn said that he would complain to the Inter-American Court of Human Rights if necessary.