Speaking via live video link from Kiev to a House of Representatives hearing in Washington, Yevgenia Tymoshenko also warned that Ukraine's powerful current rulers will do "anything possible" to manipulate results of the country's upcoming October election.
"I am very afraid for my mother's life now in the hospital," where she is surrounded by medical personnel who are under intense pressure from the regime, Yevgenia told the so-called Helsinki Commission on security and cooperation in Europe.
The opposition leader has been struggling with medical problems and complained over rough handling by her guards.
She briefly went on a hunger strike to protest her detention.
"We are just asking you please to keep the pressure on," Yevgenia said.
"Please don't leave us alone, because in Ukraine there's not enough power in people, not strong enough to fight against this injustice."
Tymoshenko has been imprisoned since August, when she was given a seven-year sentence for abuse of power.
The case has drawn international outrage with observers saying her prosecution was politically motivated.
President Viktor Yanukovych said Tymoshenko and 13 former senior officials from her government had been accused of various crimes.
Yevgenia said a Ukrainian court recently delayed hearing her mother's prison sentence appeal, so her lawyer was now presenting the case to the European Court of Human Rights.
"Those who have been imprisoned absolutely must be released," said Republican Chris Smith, the commission's chairman and a fierce human rights defender.
He told Yevgenia that he was hoping to soon bring a congressional delegation to Kiev, and that he was expecting the United Nations to "robustly" weigh in on Tymoshenko's detention.
The former premier recently had a chance to speak with US officials, her daughter said.
"My mother outlined the critical situation that she is in, legally, politically, medically and in a humanitarian way," Yevgenia said.
She added that her mother had strong worries about the upcoming election.
"The regime has accumulated so much financial power via different schemes (and) they will use this financial resource to do anything possible to falsify the elections," she said.
Also testifying at the hearing was David Kramer, president of democracy advocacy group Freedom House, who led a delegation to Ukraine last month and met with Yanukovych and other officials.
Kramer said the October polls will be "a critical test" of Ukraine's ability to conduct elections which meet the criteria of international monitors.