Ukraine’s Prime Minister Yulia Tymoshenko’s campaign said on Tuesday it plans to legally challenge the results of the presidential runoff that opposition leader Viktor Yanukovych appears to have won.

According to Ukraine’s election commission, Mr. Yanukovych is leading in Sunday’s vote by 3.5 percentage points with only 0.03 percent of precincts left to count.

Unlike past elections in Ukraine, international monitors have praised this vote as being free and fair. The United States also praised the vote, with the U.S. Embassy hailing it Tuesday as “another step in the consolidation of Ukraine’s democracy.”

But Ms. Tymoshenko’s allies say she will not concede until appeals have run their course and recounts have taken place at a number of disputed polling stations.

“We will recognize defeat only after a decision by the courts,” said Andriy Shkil, a prominent member of the Yulia Tymoshenko Bloc in parliament. “We will appeal both the preparation and conduct of the election.”

The respected Ukrainskaya Pravda Web site and Russia’s ITAR—Tass news agency cited Ms. Tymoshenko as telling party officials that she will “never recognize” the legitimacy of Sunday’s runoff and plans to demand a third round of voting.

Ms. Tymoshenko has not yet issued any calls for mass protests against the vote like those of Ukraine’s 2004 pro-Western Orange Revolution, where she was a leading figure. On Monday she cancelled two planned news conferences but she plans a news conference later on Tuesday.

Ms. Tymoshenko’s allies say the election was marred by Viktor Yanukovych.

“A decision has been taken to challenge results in certain polling stations and to demand a recount at those stations,” said Yelena Shustik, a deputy with the Yulia Tymoshenko Bloc.

But there are signs of dissent within Ms. Tymoshenko party ranks, however.

Ukrainskaya Pravda and ITAR—Tass cite deputy speaker Mykola Tomenko, also a member of the Yulia Tymoshenko Bloc, as saying at a party meeting on Monday that Ms. Tymoshenko should accept defeat and take up her new role as the opposition.

Yanukovych’s Party of Regions, meanwhile, rejected calls for further scrutiny of the election.

“There will be no third round,” Mykola Azarov, deputy head of the Party of Regions, told parliament on Tuesday. “They are dragging us into an unnecessary war.”

On Tuesday evening, Mr. Yanukovych is due to address thousands of his supporters, who have assembled outside the headquarters of the Central Election Commission in Kiev, the capital. Mr. Yanukovych’s team say it organized the meeting to defend the results of the election.

In comments apparently directed at Ms. Tymoshenko, a top European election observer urged Ukraine’s politicians to heed the official vote tally.

“It is now time for the country’s political leaders to listen to the people’s verdict and make sure that the transition of power is peaceful and constructive,” said Joao Soares, head of the observation mission from the Organization for Security and Cooperation in Europe’s Parliamentary Assembly.

A Yanukovych victory would close a chapter in the country’s political history by ousting the pro-Western leadership of the past five years that was ushered in by the Orange protests. That government foundered due to internal divisions, fierce opposition from Russian-speaking eastern Ukraine and the collapse of Ukraine’s economy.

Russian President Dmitry Medvedev called Mr. Yanukovych on Tuesday to congratulate him on a successful election performance, the Kremlin said in a statement.

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