Dmytro Bulatov, who has led street protests against the government, said he had nails driven through his hands, was strung up by his wrists and had an ear cut off before being dumped in a forest outside Kiev.
“They crucified me, so there are holes in my hands now,” the badly wounded and shaken activist said. “Along with that, they cut off my ear, cut up my face. My whole body is a mess. You can see everything.”
He survived the attack by unknown assailants, and raised the alarm after walking to a village despite his injuries and temperatures well below freezing.
Mr Bulatov’s ordeal will provide yet more substance to allegations that the government, or its sympathisers, are operating an underground and unofficial campaign of violence and intimidation against the opposition.
Some 27 activists have been reported missing, others have said they were kidnapped and attacked and one, Yuri Verbitski, was found dead after being abducted.
Journalist Vitali Portnikov, who has fled Ukraine for his own safety, said the government had formed “death squads” similar to those seen in the bloody civil conflicts of Latin America.
“The Ukrainian authorities have integrated these death squads into law enforcement agencies,” he said. “This means real war against the people.”
Katherine Ashton, the EU’s foreign policy chief, condemned the attack and called for an end to the targeting of activists.
She said she was “appalled by the obvious signs of prolonged torture and cruel treatment” of Mr Bulatov, and described it as another incident in the “continuous deliberate targeting of organisers and participants in peaceful protests”.
Vitali Klitschko, the boxer turned opposition leader, expressed his shock over the treatment of Mr Bulatov, and, after speaking with the victim, accused Moscow of involvement.
“The leader of the kidnappers had a Russian accent,” he said. “The Kremlin had a hand in this.”
News of Mr Bulatov’s ordeal came as the situation in Ukraine remained tense with protesters still manning the barricades in central Kiev, and the prospects of a settlement to a crisis threatening to plunge the country into civil war remote.
In its first statement on the situation, the defence ministry urged president Viktor Yanukovich to take all the necessary legal steps to restore order.
“Laying out their civil position, servicemen and employees of Ukraine’s armed forces …called on the commander-in-chief to take urgent steps within the limits of existing legislation with a view to stabilising the situation in the country and reaching consent in society,” the ministry said.
So far Ukraine’s armed forces have not been involved in the violent clashes between protesters and police but would almost certainly be drawn into the crisis if the government declared a state of emergency.
US secretary of state John Kerry said the Ukrainian government had failed to make enough concessions to the opposition.
But in Moscow, Kremlin economic adviser Sergei Glazyev claimed “outside forces” had helped provoke the crisis, and he called on Mr Yanukovich to crush the opposition.