For eight days, people feared Dmytro Bulatov was dead. One of the leaders of protests against President Viktor Yanukovych, the activist disappeared without trace on 22 January. Now he has resurfaced, alive, but says he was subjected to horrific torture, including a crucifixion, while he was held in captivity by unknown men.

Bulatov was discovered in a village near Kiev late on Thursday. He said his kidnappers beat him severely, nailed him to a cross, sliced off a piece of his ear and cut his face. Bulatov, 35, is part of a group of car owners who have taken part in the protests against Yanukovych, blocking police and troop movements with their vehicles in an attempt to stymie the government response.

"They crucified me, they nailed down my hands … There isn't a spot on my body that hasn't been beaten," he said on local television. Bulatov's face and clothes were covered in clotted blood, and his hands were swollen and bore the marks of nails.

Bulatov is now undergoing treatment for his injuries, and doctors were not letting journalists visit him in hospital on Friday. However, in a video address posted on his Facebook page by a friend, he told of what had happened to him: "I was brutally beaten, had a bag on my head, and was subjected to very severe tortures, but nevertheless they will not be able to intimidate us and we are not going to stop."

His friend and fellow activist Oleksiy Hrytsenko wrote: "He is keeping well despite the fact that these bastards applied all kinds of torture to him."

Vitali Klitschko speaks to Dmytro Bulatov in hospital. Photograph: AP

Bulatov's reappearance comes at a key time in Ukraine's political crisis, as Yanukovych signed into law a conditional amnesty for those involved in violence over recent weeks, but the protesters that have occupied central Kiev say they will accept nothing less than snap elections. On Friday, the Kremlin urged the Ukrainian leadership to crack down on the protest movement.

Both Bulatov and Hrytsenko are on a wanted list for organising mass disturbances, and a spokesperson for the interior ministry told Ukrainian media he believed it was possible that Bulatov had staged his own kidnapping: "This could have been staged as a provocation, in order to create negative emotions in society," said Oleg Tarasov. He added it was also possible that Bulatov had been kidnapped over a financial issue.

The opposition dismissed these theories as nonsense and linked Bulatov's kidnapping to a whole spate of sinister disappearances. The case is reminiscent of the abduction of two other activists – Igor Lutsenko and Yuri Verbytsky – who were kidnapped from a hospital last week. Lutsenko said both men were driven to a forest where they were interrogated and beaten. Lutsenko managed to crawl out of the forest and make it to a hospital. Verbytsky was later found dead in the forest, his hands tied behind his back and a bag on his head.

"They behaved during the interrogation like people who have been doing this for many years," Lutsenko said last week about the men who captured and tortured him.

"What happened to Dmytro is an act of intimidation to all the protesters," said opposition leader Vitali Klitschko after visiting Bulatov in hospital.

Protesters warm themselves up on the barricade during ongoing protests in Kiev. Photograph: Filip Janek

The protests in Ukraine started after Yanukovych backed out of an agreement to deepen ties with the European Union, but the demonstrations quickly came to encompass an array of discontent over corruption, heavy-handed police and a dubious justice system.

Yanukovych on Friday signed into law an amnesty for those detained during the unrest and approved the repeal of anti-protest legislation. But demonstrators have rejected the amnesty because it is conditional on occupied buildings being cleared of activists.

Hardline prime minister Mykola Azarov has resigned, but opposition leader Arseniy Yatsenyuk declined the offer to serve as prime minister under a Yanukovych presidency. For now, Serhiy Arbuzov, a member of Yanukovych's inner circle, is serving as interim PM. Yanukovych himself went on sick leave this week, due to an acute respiratory illness and high fever, which aides say could be caused by stress.

Moscow has said it will hold off a planned $2bn purchase of Ukrainian government bonds until a new government is formed. In strong words yesterday, the Kremlin's point man on Ukraine warned that Yanukovych would lose power unless he "quashed the rebellion".

"He is currently in the situation of a creeping coup and because he is the guarantor of the constitution, security and integrity of Ukraine, then the president has no choice," said Kremlin economic aide Sergei Glazyev.

Viktor Yanukovych. Photograph: Janek Skarzynski/AFP/Getty Images

"Either he defends Ukrainian statehood and quashes the rebellion provoked by financial and outside forces or he risks losing power, and mounting chaos and an internal conflict, from which no exit can be seen, await Ukraine."

On Friday John Kerry, the US secretary of state, said that overtures by Yanukovych to the opposition have not been enough to resolve the crisis.

Kerry, who is set to meet key opposition figures including Yatsenyuk and Klitschko on the sidelines of a security conference starting in Munich on Friday, told a news conference: "The offers of President Yanukovych have not yet reached an adequate level of reform and an adequate level of sharing of the future so that the opposition can, in fact, feel that it can legitimately come to the table."

But he added that if the government presents a reform agenda offering "genuine participation" then the opposition should seize the opportunity "because further violence that goes out of control is not in anybody's interest".