Ukrainian police have opened an investigation into the kidnapping of an opposition activist, who said he was held captive for more than a week and tortured.

It was the latest in a string of mysterious attacks on anti-government protesters in the two-month-long political crisis.

Dmytro Bulatov, 35, a member of Automaidan, a group of car owners that has taken part in the protests against President Viktor Yanukovych, went missing on January 22.

He was discovered outside Kiev on Thursday. He said his kidnappers beat him badly, drove nails against his hands, sliced off a piece of ear and cut his face. He was kept in the dark all the time and could not identify the kidnappers. After more than a week of beatings, they eventually dumped him in a forest.

"They crucified me, they nailed down my hands. They cut off my ear, they cut my face. There isn't a spot on my body that hasn't been beaten," Mr Bulatov said on Channel 5 television. "Thank God, I am alive."

His face and clothes were covered in clotted blood, his hands were swollen and bore the marks of nails.

Opposition leader Petro Poroshenko rushed to the hospital where Mr Bulatov was taken on Thursday night.

"Dmytro asked to pass his greetings to everyone and to say that he has not been broken and will not be broken," a grim-looking Mr Poroshenko said. "That he is full of energy and despite the fact that his body was been beaten, Dmitry's spirit is strong."

Police said the car he was driving when he disappeared had been found.

He had been missing for eight days, and the protesters organised a campaign for his release. They pleaded with top government officials for assistance, offered a 25,000 dollar (£15,000) bounty to anyone who could help locate him and even consulted psychics, said Oleksiy Hrytsenko, Mr Bulatov's friend and fellow activist.

Mr Hrytsenko said Automaidan members had come under tremendous pressure during the protests, with their cars burnt and activists detained, harassed and threatened. He showed a reporter a text message he had received from an unknown number that read: "Go ahead, go ahead, your mother will be happy to see her son dead."

Mr Bulatov is among three activists whose disappearances have shocked the country, especially after one of them was found dead.

He went missing one day after Igor Lutsenko, another prominent opposition activist who had also gone missing, was discovered after being taken to the woods and beaten severely by unknown attackers.

Mr Lutsenko was kidnapped from a hospital, where he had brought a fellow protester, Yuri Verbitsky, to be treated for an eye injury. Mr Verbitsky was also beaten severely and later discovered dead.

The disappearances prompted an outcry from protesters, who accused the government of intimidating the opposition.

The protests started after Mr Yanukovych backed out of an agreement to deepen ties with the European Union in November, but quickly came to include an array of discontent over corruption, heavy-handed police and dubious courts.

Negotiations between the authorities and the opposition on finding a way out of the crisis appeared to have stalled on Thursday, after Mr Yanukovych took an unexpected sick leave and told opposition leaders that it was now up to them to make concessions.

This week Mr Yanukovych accepted the resignation of Prime Minister Mykola Azarov and the parliament, which he controls, and rescinded harsh anti-protest laws that sparked last week's violence. But a bill passed by Mr Yanukovych's allies in parliament offered to grant amnesty to protesters only after they vacate scores of government buildings they have seized across the country, a demand rejected by the opposition.