Dmytro Bulatov, 35, a member of Automaidan, a group of car owners that has taken part in the protests against President Viktor Yanukovych, went missing on January 22.
He was discovered outside Kiev on Thursday night and said his kidnappers beat him badly, drove nails against his hands, sliced off a piece of an ear and cut his face.
The activist said: "They crucified me, they nailed down my hands. They cut off my ear, they cut my face. There isn't a spot on my body that hasn't been beaten."
EU foreign policy chief, Catherine Ashton, said she was "appalled by the obvious signs of prolonged torture and cruel treatment" of Mr Bulatov.
Meanwhile, Mr Yanukovich has signed into law an amnesty for demonstrators detained during recent unrest and repealed anti-protest legislation despite being on sick leave.
The 63-year-old leader, who looks increasingly isolated in a tug-of-war between the West and Ukraine's former Soviet overlord Russia, suddenly withdrew from view on Thursday, complaining of a high temperature and acute respiratory ailment.
At least six people have been killed and hundreds more injured in two months of street battles between anti-government demonstrators and police, which have escalated sharply after the authorities toughened their response. The crisis forced Prime Minister Mykola Azarov to resign earlier this week and as yet there is no sign of a successor. Serhiy Arbuzov, Mr Azarov's first deputy and a close family friend of Mr Yanukovich, has stepped in as interim prime minister.
US Secretary of State John Kerry said yesterday: "We offer Ukraine's opposition the full support of President Obama and of the American people for their efforts. But if you get that reform agenda we would urge them to engage in that because further stand-off, or further violence that becomes uncontrollable, is not in anybody's interests."