Ukraine opposition activist Dmytro Bulatov has outlined details of his imprisonment and torture, saying his captors cut off part of his ear before touching it to his face.

Bulatov described his beatings as "very professional" while speaking to reporters on Thursday in Lithuania's capital Vilnius, where he headed for medical treatment after being discovered eight days after he went missing on January 22.

"They cut my ear, I couldn't feel how much they cut off my ear. They took the piece of my ear and touched my face with it," Bulatov said, adding that he became so desperate as the beatings continued that he falsely confessed to having received $50,000 from the US Ambassador in an attempt to stop the torture.

I was asking them to kill me because I was unable to stand this any more. Some time later they came and put a bag on my head and took me somewhere. I was hoping my suffering was finished. Dmytro Bulatov, oppositon leader

Bulatov said his captors were continually asking who was funding the protests in Ukraine and accused him of being a US spy paid by the CIA.

"It felt as if melted metal was poured on my face. The only thing I wanted was for this to stop," Bulatov told reporters. "I said whatever they wanted."

Automaidan, the protest group run by Bulatov, was largely made up of drivers who would blockade streets to protect protest camps.

On the final day, Bulatov says he was pinned to a wooden door. "On the last day they put me on my knees. They said 'we are going to crucify you,'" he said.

Bulatov says that nails were then driven through his hands.

"I was asking them to kill me because I was unable to stand this any more. Some time later they came and put a bag on my head and took me somewhere. I was hoping my suffering was finished."

Al Jazeera's Nadim Baba, reporting from Vilnius, said there are conflicting accounts about what happened to Bulatov and that the Ukrainian government has cast doubt on his story.

Our correspondent said Bulatov hinted that those who abducted him might be from the Russian security services.

"He said that was because they were speaking Russian 'with an accent' - his words. He says he wasn't sure but that's where his suspicions lie. Obviously that's going to be very incendiary for his supporters in Ukraine."

'Attempt to overthrow'

His detention became a rallying point for Ukrainian protesters in Kiev and was condemned by EU foreign policy chief Catherine Ashton who called it a "deliberate targeting" of peaceful protesters.

The identities of those who took him are not known.

Late on Wednesday, opposition leader and former interior minister, Yuriy Lutsenko, said the country's security services are investigating him for an alleged attempt to "overthrow the state".

Lutsenko, a close ally of jailed pro-Western former Prime Minister Yulia Tymoshenko, is a key figure in the protest movement locked in a two-month confrontation with Ukraine's President Viktor Yanukovych.

"Today I received notice from the SBU security service about a case against me over "an attempt to overthrow the constitutional state'," Lutsenko said in remarks published on his official website.

He said the investigation was linked to a call he made for the creation of an alternative parliament to be established under the name People's Rada.

If convicted the 49-year-old politician, who spent two years in jail before being released last year under Western pressure, could serve up to five years in prison.

No official confirmation of the investigation was immediately available.