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Dmytro Bulatov, 35, one of the leaders of anti-government protest motorcades called 'Automaidan', turned up on Thursday (local time) with his face badly beaten and with wounds to his hands.
During a week of being confined, he was tortured by his kidnappers, who had "crucified" him, he told Ukraine's Channel 5 television.
Bulatov is on a police wanted list on suspicion of taking part in "mass disorder" - which carries a sentence of up to eight years - linked to participation in the motorcades.
Police said they went to the Kiev clinic where Bulatov was being treated for his injuries and a police statement later said Bulatov was under police protection.
Opposition leaders, including boxer-turned-politician Vitaly Klitschko, said however that police had intended to arrest him but had been thwarted by doctors who quickly called opposition leaders to the scene.
"The police are trying to provoke further escalation of the conflict and increase tension in society. Instead of searching for those guilty for the disappearance and torture of one of the leaders of the 'automaidan'," Klitschko's UDAR (Punch) party said in a statement.
"The police are trying to make a criminal of him."
Attempts to arrest Bulatov had been thwarted by the quick response of doctors, another opposition leader, far-right nationalist Oleh Tyahnibok, told reporters.
Opposition deputies would take shifts to protect Bulatov against the police overnight, he said.
Bulatov, whose disfigured and swollen face was replayed on TV screens throughout the day, said: "They crucified me. They punctured my hands," and he pointed to marks on the backs of his hands. "They cut off my ear, slashed my face.
"But I am alive, thank God."
In Brussels, European Union foreign policy chief Catherine Ashton said she was said she was appalled by signs of torture inflicted on Bulatov. "All such acts are unacceptable and must immediately be stopped," she said in a statement.
Bulatov was reported missing on Jan. 23. He was involved in several motorcade protests in which scores of cars would drive to the homes of Ukrainian leaders. The 'automaidan' is a word play on Maidan, the Kiev square that is a focus for revolt.
In the biggest motorised protest, about 2,000 cars drove to the country residence of President Viktor Yanukovich at Mezhyhirya, outside Kiev, on Dec. 29. They came within 300 metres (yards) of his residence before being stopped by security roadblocks.
Traffic police have begun to try to identify participants in the 'automaidan' protests and protest groups say that about 20 people so far have been detained for questioning.
News of Bulatov's kidnapping and injuries emerged ahead of another weekend of protest rallies in Kiev though organisers have discouraged people from turning out because of a cold spell that has sent night temperatures plunging to 18 Celsius (30 Fahrenheit) below freezing.
The protests began after Yanukovich rejected an EU trade deal last November in favour of closer ties with Moscow and a financial bailout from Russia. They have since spiralled into a public show of anger against perceived misrule and corruption.