The graphic images of a Ukrainian opposition leader, who says he was abducted and tortured for more than a week, have raised fears that extrajudicial squads are being deployed to intimidate protesters.
Dmytro Bulatov, 35, who re-emerged on Friday after being missing for a week, said unidentified kidnappers beat him, sliced off part of his ear and nailed him to a door during his time in captivity.
"There isn't a spot on my body that hasn't been beaten. My face has been cut. They promised to poke my eye out. They cut off my ear," Bulatov said on Friday from a hospital in Kiev.
"They crucified me by nailing me to a door with something and beat me strongly all the while."
|Latest updates on Ukraine's alleged torture victim Dmytro Bulatov
The Interior Ministry says it is investigating Bulatov's story, but it has also accused him of failing to cooperate.
Oleh Tatarov, deputy chief of the ministry's main investigative department, said Bulatov's kidnapping could have been staged in order to create a provocation.
Later on Friday, the ministry dispatched investigators to Bulatov's hospital to interrogate him, saying that besides being a kidnapping victim, he was also suspected of organising mass disorders in the protests.
Opposition legislators and Bulatov's supporters feared he was about to be arrested and rushed to the hospital to shield him from police.
The standoff continued late into the evening.
The government has faced two months of major protests that started after President Viktor Yanukovich backed out of an agreement to deepen ties with the EU in favour of Russia.
The demonstrations quickly grew into discontent over heavy-handed police, corruption and human rights violations.
In another development on Friday, Ukraine's security service announced a criminal inquiry into an opposition attempt to seize state power, after information on confiscated computers allegedly revealed the country's mass protests were "pre-planned".
"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 Yulia Tymoshenko, the jailed former prime minister.
Against this backdrop, the Ukrainian opposition urged Europe and the US to go beyond vocal support for their fight for more democracy and demand a halt to violence they blame on Yanukovich.
Opposition leaders held a meeting in Munich, Germany, on Friday with Frank-Walter Steinmeier, the German foreign minister, and Catherine Ashton, the EU foreign policy chief, and were due to meet John Kerry, the US secretary of state, on Saturday.
"What we need is not just declarations but a very clear action plan - how to fix the problem and fix the violence, how to investigate all these killings and abductions and tortures," Arseniy Yatsenyuk, leader of Batkivschyna, told Reuters news agency.
Ashton issued a statement on Friday saying she was "appalled by the obvious signs of prolonged torture and cruel treatment" of Bulatov.
For its part, the Obama administration said it is "appalled" by signs of torture against Bulatov.
Jay Carney, the White House spokesman, said the US administration is concerned by increasing reports of protesters disappearing and being beaten and tortured, and by attacks on journalists.
Carney also said it is "especially concerning" that some of the reports suggest that Ukrainian security forces have been involved.
Bulatov's group, car owners known as Automaidan, started out by picketing the residences of top government officials and their allies, but soon took an active part in the protests that have rocked Ukraine.
The group, which took its name from Kiev's Independence Square - known as Maidan - blocked streets and monitored police cars.
|Unrest spreads across Ukraine
Bulatov went missing on January 22, prompting his friends to organise a campaign for his release.
They pleaded with top government officials for assistance, offered a $25,000 bounty to anyone who could help locate him and even consulted psychics, fellow activist Oleksiy Hrytsenko said.
Hrytsenko grew all the more worried about Bulatov's fate because Automaidan members' cars were being torched and their activists detained, harassed and threatened.
Hrytsenko showed an Associated Press reporter a text message he received from an unknown number that read: "Go ahead, go ahead, your mother will be happy to see her son dead."
Bulatov was dumped in a forest on Thursday night after eight days in captivity and made it to a house outside Kiev where he got help and was able to call friends, according to an Interior Ministry statement.
The reported beatings and intimidation have stoked speculation that special security teams were roaming Ukraine and hunting down opposition activists.
President Yanukovich took indefinite sick leave on Thursday, stalling the negotiations between authorities and the opposition in a bid to find a way out of the political crisis.
He told opposition leaders it was now up to them to make concessions, since he already accepted the resignations of Mykola Azarov as prime minister, and approved the scrapping of harsh anti-protest legislation that led to last week's violence.
Yanukovich also signed into law a bill offering to grant amnesty to protesters, but only after they vacate scores of government buildings they have seized across the country.