Yulia Tymoshenko, Ukraine's former prime minister, has gone on a hunger strike, alleging that guards in the jail where she is being held had severely beaten her.
Tymoshenko announced the hunger strike on Tuesday, saying she had refused food since Friday night in order to attract "the attention of the democratic world to what is happening in the centre of Europe, in a country called Ukraine".
Prosecutors acknowledged that prison guards had taken Tymoshenko to a nearby hospital for treatment of an existing medical problem against her will, but insisted she was not beaten.
Tymoshenko, the country's top opposition leader, is serving a seven-year prison term on charges of abusing her powers during negotiations about gas supplies with Russia.
"Three strong men came up to my bed, threw a sheet on me and then started pulling me off the bed by force. In pain and despair, I started defending myself as I could, and was hit hard with a fist in the stomach," she said in a statement on her website.
"They tied up my arms and legs, lifted me up and dragged me out in the sheet," she said. "I thought the last minutes of my life had come."
Last week, her back pain kept her from appearing for the start of a new trial on tax evasion charges that could see her jail term extended from seven to 12 years.
She has asked to have treatment abroad, saying she fears that local doctors could poison her or infect her with a disease.
Ukrainian authorities have been been accused of involvement in the poisoning of former president Viktor Yushchenko, who was Tymoshenko's close ally during the 2004 Orange Revolution.
Yushchenko's face became disfigured in 2004 during his fight for disputed polls won by current President Viktor Yanukovych to be declared invalid.
The West has strongly condemned Tymoshenko's imprisonment as politically motivated and threatened to freeze co-operation with Ukraine.
Tymoshenko denies the charges against her, saying they are part of a campaign by Yanukovych to bar her from politics.
A European Commission spokesman demanded urgent "clarification" from the Ukraine authorities after Tymoshenko's statement on Tuesday.
"We are urgently seeking clarification from the Ukrainian authorities on the situation of Mrs Tymoshenko and insisting that appropriate independent persons, and her legal advisers, should immediately have access to her for a sufficient duration," the spokesman said.
Prosecutors claimed that prison officials had acted within the law and insisted there was no evidence to support the allegations of beating.
"According to the law ... the prison service has the right to use physical measures," Henadiy Tyurin, a Kharkiv regional prosecutor, told reporters.
"She was picked up, carried to the car and taken to the hospital."