Ukrainian opposition leaders are due to meet the US secretary of state, John Kerry, a day after the US condemned the apparent torture of a protest leader who claims unidentified abductors "crucified" him.
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," he said on Friday. "They crucified me by nailing me to a door, and beat me."
Opposition leaders were due on Saturday to meet Kerry in Munich after the US said it is "appalled" by signs of torture against Bulatov.
Jay Carney, the White House spokesman, said the US administration was concerned by increasing reports of protesters disappearing and being beaten and tortured, and by attacks on journalists.
|Ukrainian protest leader found alive
Carney also said it is "especially concerning" that some of the reports suggest that Ukrainian security forces have been involved.
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 the president, Viktor Yanukovich.
"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," said Arseniy Yatsenyuk, the leader of Batkivschyna said on Friday.
He added: "Ukraine desperately needs a Marshall Plan and not martial law in order to stabilise the political and economic situation in the country."
On Saturday, the president of the EU's European Council, Herman von Rumpoy, said that the future of Ukraine blongs with the EU".
Russia, Yanukovich's main international ally, accused the EU of double standards, saying none of the member states would tolerate violent protests on their streets.
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.
Bulatov went missing on January 22, prompting his friends to organise a campaign for his release.
He 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.
|Latest updates on alleged torture victim Dmytro Bulatov
The reported beatings and intimidation have stoked speculation that special security teams were roaming Ukraine and hunting down opposition activists.
The government has faced two months of major protests that started after Yanukovich reneged on 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.