Former Ukrainian President Viktor Yushchenko testified against ex-Prime Minister Yulia Tymoshenko in her abuse of power trial in Kyiv Wednesday, while her supporters chanted “Shame! Shame!”

Mr. Yushchenko testified that the natural gas agreement Ms. Tymoshenko signed with Russia in 2009 was a bad financial deal for Ukraine, saying that Ms. Tymoshenko traded national interests in favor of political gain.

Ms. Tymoshenko, who was charged with abuse of power and was arrested and jailed for contempt of court on August 5, has denied the charges. She says they were orchestrated by current Ukrainian president Viktor Yanukovich in order to keep her out of politics.

Ms. Tymoshenko is expected to be the main opposition candidate in next year's election. She will not be eligible to run if she is convicted.

The gas deal, which Mr. Yushchenko says was signed at an inflated price, ended a price-war that had temporarily cut off Russian gas supplies to Ukraine. Mr. Yushchenko said that Ms. Tymoshenko, who was preparing to run in presidential elections the following year, wanted to be seen as a “savior” for ending the price dispute.

The U.S. State Department and the European Union have said Ms. Tymoshenko's arrest appears to be politically motivated and have urged that she be released. But Ukrainian President Yanukovich has refused to intervene, calling such action “unacceptable' and adding that Ukraine's court system is independent and unbiased.

The former prime minister is one of about 400 officials under investigation for crimes allegedly committed while in office. Critics say the only cases that have come to trial involve political figures opposed to President Yanukovich.

Ms. Tymoshenko rose to fame in 2004 as a leader of Ukraine's “Orange Revolution” street protests, which forced the Supreme Court to toss out the results of a disputed presidential election in which Mr. Yanukovich was initially declared the winner. The two competed in last year's presidential election, which Mr. Yanukovich won.