Ukraine's Supreme Court debated the validity of presidential election results Monday, while an eastern province scheduled a referendum on autonomy and the opposition threatened to further paralyze the government through a blockade.

The court was considering an appeal by opposition candidate Viktor Yushchenko (search) against the bitterly disputed results of the Nov. 21 presidential runoff, which declared Kremlin-backed Prime Minister Viktor Yanukovych (search) the winner.

Under Ukrainian election legislation, the court is unable to rule on the overall results but can declare results invalid in individual precincts. Mykola Katerinchuk (search), an aide to Yushchenko, said the appeal focused the results in eight eastern and southern Ukrainian regions — more than 15 million votes, almost half of the total number in the runoff.

The Western-leaning opposition claimed "severe violations of Ukrainian legislation" and asked the court to throw out the results, he said.

The court was expected to hear arguments, then retire to review the case before issuing a decision. It was not clear how long the proceedings would last.

The ruling could pave the way for a new vote, which the opposition is demanding, or remove the only barrier to the inauguration of Yanukovych, who has the backing of outgoing President Leonid Kuchma and the Kremlin, which still yields significant political and economic influence over energy-dependent Ukraine. Yanukovych was declared the winner with a margin of 871,402 votes.

"The official results of the elections do not meet the people's will and this is a violation of their constitutional rights," Roman Zvarych, a Yushchenko aide, said inside the courthouse. "I hope that the Supreme Court will be guided by the law."

While the court's decision is likely to boost the legitimacy of whichever side it seems to favor, it could also deepen the divide and prolong the crisis by fueling anger in the other camp. Thousands of pro-Yushchenko and pro-Yanukovych supporters massed outside the court building.

But tension mounted ahead of the session. Yushchenko, who claims his victory was stolen through election fraud, rejected government appeals Sunday to call off tens of thousands of protesters and urged his backers to maintain their weeklong round-the-clock vigil and their blockades of the Cabinet building and the presidential administration.

His supporters refused to let anyone but security personnel enter the buildings, and some Cabinet ministers have complained the blockades are impeding the state's work.

Yushchenko's more radical ally, Yuliya Tymoshenko, threatened Kuchma with an ultimatum Sunday, saying opposition supporters would block his movements if he didn't fire Yanukovych and governors of eastern regions threatening to push for self-rule, by late Monday.

"We know where he is, and we can prevent him from making a single step if he doesn't fulfill our demands," Tymoshenko told a crowd of some 100,000 at Kiev's central Independence Square.

Yanukovych supporters struck back from the city of Donetsk, the prime minister's native region and main power base. The regional legislature voted 164-1 to hold a Dec. 5 referendum on autonomy for the province. About 30,000 demonstrators gathered outside the legislature building there.

"We won't tolerate what's going on in Ukraine," said Donetsk governor Anatoly Bliznyuk. "We have shown that we are a force to reckon with."

The referendum would ask Donetsk voters to demand the status of a republic for the region, which would require changing Ukraine's constitution to allow for stronger self-rule for its provinces. While such changes could face serious opposition, the vote suggested Ukraine's rift could deepen much further if the election results are overturned.

The referendum move came after an urgent meeting attended by Yanukovych and some 3,500 delegates from the east and south who adopted a resolution threatening to hold a similar plebiscite if the crisis worsens.

Borys Kolesnikov, the speaker of the Donetsk regional legislature, warned that a Yushchenko presidency could lead to eastern and southern provinces splitting away and forming an autonomous southeastern republic with its capital in Kharkiv, close to the Russian border.

The crisis has deepened the divide between Ukraine's pro-Russian east and Yushchenko's strongholds in the capital and western Ukraine, which is a traditional center of Ukrainian nationalism.

The Supreme Court said last week that the election's official results could not be published until it rules on Yushchenko's challenge. Yanukovych cannot formally take office until the results are published. On Saturday, Ukraine's parliament passed a nonbinding resolution declaring the election invalid.

Russia and the West are at odds over the political stalemate in Ukraine. Russian President Vladimir Putin openly backed Yanukovych and congratulated him on his victory, while many Western nations say have not recognized the official results.

Meanwhile, the head of Ukraine's Central Bank, Serhiy Tyhypko, said Monday that he had resigned to engage in politics full-time, the Interfax news agency reported. Tyhypko, a former economics minister, had taken leave from his work since the summer to head Yanukovych's presidential campaign. On Sunday, Kuchma ordered him to end his leave and go to work at the Central Bank.