A court Tuesday sentenced Tymoshenko to seven years in jail for abuse of power in connection with a 2009 gas deal she helped broker with Russian gas company Gazprom.
Her Western supporters argued that the case against Tymoshenko was politically motivated. The former leader of the Orange Revolution has since moved to the opposition against President Viktor Yanukovych.
The verdict comes as Kiev, under Yanukovych, tries to move closer to the European Union. Jerzy Buzek, president of the European Parliament, issued a strong statement condemning the sentence.
"Strengthening the rule of law and a credible fight against corruption are essential not only for the conclusion of the association agreement by the end of the year and the deepening of EU-Ukraine relations but also for the consolidation of democracy in Ukraine," he said.
Several members of the European Parliament said Brussels should call off talks with Kiev because of the verdict. Buzek added, however, that he hoped the case would be overturned on appeal.
Tymoshenko, in a statement posted on her Web site, said justice was being "trampled" by the Ukrainian court.
"I'm certain that the European court will cancel this unjust ruling," she said after the ruling.
Moscow questioned the ruling. Tymoshenko, the Russian Foreign Ministry said in a statement published by state-run news agency RIA Novosti, "was prosecuted for the current, still valid, legally binding agreements between Gazprom and Naftogaz Ukraine."
The verdict can be appealed within15 days, the court said.