Dmytro Bulatov, who organized car protests for the opposition camped out in Kiev, is being treated in hospital after being found near the capital.
Police have confirmed Dmytro Bulatov, 35, received an ear injury and bruising.
In another development, the army called on President Viktor Yanukovych to take “urgent steps” to ease the crisis.
Three protesters and three police officers have been killed, and scores injured on both sides, since the protests turned violent on January 22.
Opposition to Viktor Yanukovych spilled into the streets in November after he abandoned a trade deal with the EU in favor of closer economic ties with Russia.
President Viktor Yanukovych accused the opposition of seeking to “inflame” the situation on Thursday by continuing the protests despite moves by the government and parliament to ease the stand-off.
Parliament voted to annul a recently enacted law restricting protests and passed a law giving amnesty to detained protesters, under the condition that occupied buildings were vacated.
Anti-government demonstrators remain in their camp in Independence Square (Maidan) with no sign of the political crisis in the country coming to an end.
President Viktor Yanukovych, 63, has gone on sick leave, with his staff reporting he has a respiratory illness and a high fever.
Dmytro Bulatov says he was left to die by his captors after being kidnapped, repeatedly beaten and “crucified”. He was, he said, hung up by his wrists.
“They crucified me, so there are holes in my hands now,” he said.
“Other than that – they cut off my ear, cut up my face. My whole body is a mess. You can see everything. I am alive. Thank God for this.”
Dmytro Bulatov reportedly said he did not know who had abducted him but his abductors had spoken with Russian accents.
According to the Ukrainian news website Gazeta.ua, doctors found no damage to his internal organs or his skull.
Police in Kiev have confirmed Dmytro Bulatov was bruised and received a cut to one of his ears, Ukrainian newspaper Ukrainskaya Pravda reports.
Police have opened an investigation and posted guards at the hospital where he is being treated.
According to the paper, they also expressed indignation that Dmytro Bulatov had not phoned them after his release.
Dmytro Bulatov is a prominent anti-government activist as one of the leaders of the organization Automaidan, a group that has patrolled streets around Independence Square.
It has also driven in convoys to protest outside government ministers’ homes.
Vitali Klitschko, one of the most prominent leaders of the protesters, visited Dmytro Bulatov in hospital.
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