Leading Ukraine dissident beaten and tortured

"An investigation for an attempted takeover of power has been opened," Maxime Lenko, head of the investigations department of the Ukrainian Security Service (SBU), was quoted as saying by Interfax news agency.

The probe comes after investigators examined information stored on computer servers that were seized in December during a raid on the Kiev headquarters of the opposition Batkivschyna party, linked to jailed former premier Yulia Tymoshenko.

Information on the servers showed that the mass anti-government protests that have plunged Ukraine into its worst crisis since independence were "pre-planned", as was "the use of force against protesters... to undermine the authority of the president," Lenko said. The unrest in Ukraine erupted after President Viktor Yanukovych rejected a key EU deal in November in favour of closer ties with Moscow, bringing hundreds of thousands of pro-EU protesters into the streets.

The protests have at times turned violent and this week led to the resignation of the prime minister and the entire cabinet.Meanwhile medical workers have blocked police from questioning a Ukrainian protester who says he was abducted and tortured.

Dmytro Bulatov, 35, who went missing during anti-government protests was rescued after he stumbled into a village near Kiev.

"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," he said.

Bulatov, a member of Automaidan, a group of car owners that has taken part in the protests against President Viktor Yanukovich, was reported missing on January 22.

He is among several activists whose disappearances have increased tensions the country.

One of the activists, Yuriy Verbytsky, was found dead in a forest while another, Igor Lutsenko, survived a severe beating and was hospitalised.

Opposition leader Vitali Klitschko, who visited Bulatov in hospital, said: "What was done to Dmytro was an act to frighten all citizens who are being active now."

The UN's human rights office has called on Ukraine to launch an independent investigation into deaths, kidnappings and torture during the weeks of political unrest. Rupert Coville, spokesman for the UN high commissioner for human rights, said the issue "should be promptly, thoroughly and independently investigated".

The UN statement came as Russia's outspoken Deputy Prime Minister Dmitry Rogozin tweeted that US Secretary of State John Kerry's planned meeting with Mr Klitschko was a "circus".