Supporters of opposition leader Viktor Yushchenko (search) lost a round to their foes in parliament Saturday, when pro-government lawmakers helped block legal changes intended to prevent fraud in the Dec. 26 repeat of the country's presidential runoff vote.

Buoyed by a momentous Supreme Court ruling a day earlier that ordered the new vote, Yushchenko's supporters vowed to force parliament to adopt the legislation by continuing their marathon protest in Kiev's central square.

Outgoing President Leonid Kuchma (search) tried to bring European pressure on the opposition, which he accused of reneging on its promises. Kuchma called for a new round of European-mediated talks with the opposition on Monday.

The jockeying for position came as the opposition celebrated its court victory, confident Yushchenko can beat Prime Minster Viktor Yanukovych if a fair runoff is held. Yanukovych, Kuchma's ally, confirmed he would compete in the new vote, and the Central Election Commission formally set a Dec. 26 date for the balloting, as ordered by the court.

Yushchenko had been hoping for quick approval of a raft of bills that would amend election laws and reshuffle the election commission, which the opposition accused of covering up fraud in the initial runoff, held Nov. 21. The opposition says the changes are needed to ensure the new vote is fair.

But the package was blocked when a compromise agreement fell apart, and the parliament adjourned for 10 days without passing the legislation.

Communists, socialists and pro-government factions in parliament had promised to back the legislation in exchange for the opposition's support of a constitutional reform bill that would transfer some of the president's powers to parliament.

Late Saturday, Yushchenko told thousands of supporters gathered in Kiev's main Independence Square that his government foes were trying to trim presidential powers, fearing his victory. He urged the global community to help monitor the new vote.

"They realized that they aren't going to win, so they decided to make constitutional changes to revise presidential powers," Yushchenko said.

"Despite Christmas, the international community must ensure strong observers' presence," he said at the rally. "That will be the day that will determine Ukraine's fate for decades and centuries ahead."

The chairman of the Organization for Security and Cooperation in Europe, Solomon Passy, pledged that the group would again deploy a full-fledged mission for the rerun.

The opposition, meanwhile, warned that thousands of protesters could force the parliament to change course.

Yushchenko's supporters objected to the proposed changes, insisting instead that the parliament vote on electoral amendments now and consider the constitutional reforms after the presidential rerun.

"Combining electoral changes and constitutional reform is legally illiterate," said lawmaker Yulia Tymoshenko, a fiery Yushchenko ally. "We must first create legal conditions for holding elections. No one in the world passes constitutional changes in a rush."

Socialist leader Oleksandr Moroz accused Yushchenko of reneging on his promise. "No one is going to win from reneging on the agreement," Moroz said.

Kuchma, who has strongly pushed for the constitutional reform, also accused the opposition of breaking its promises.

"The opposition isn't fulfilling practically any of the agreements reached at a round table that involved European politicians," Kuchma said in a phone conversation with Dutch Prime Minister Jan Peter Balkenende, whose country holds the rotating presidency of the European Union. "That exacerbates the situation in the country."

Wednesday's compromise agreement, which was sponsored by EU foreign policy chief Javier Solana, Polish President Aleksander Kwasniewski and other mediators, called for parliament to vote for the electoral and constitutional changes all at once.

Kuchma also spoke Saturday with Russian President Vladimir Putin, who promised to send Russia's parliament speaker to attend Monday's European-sponsored talks. Putin has staunchly backed Yanukovych and scoffed at the repeat of the runoff.

U.S. Secretary of State Colin Powell hailed the court-ordered rerun as a "a victory for the Ukrainian people."

"We hope that the upcoming election will be free, fair, open, well-supervised," Powell said in Washington.

A spokeswoman for Yanukovych said that the prime minister had decided to participate in the rematch. Some observers had speculated Yanukovych, dispirited by the court ruling, might bow out.

"There's no other way but to participate and win," Yanukovych spokeswoman Anna Herman told journalists.

Yushchenko's supporters maintained a blockade of government buildings and thousands massed outside the parliament building Saturday, breaking into regular chants of "Yushchenko!"

Dmytro Klishevych, 27, waved a Ferrari flag with an orange ribbon attached to it. "I want the victory to be as fast as a Ferrari," he said.

Activists vowed to keep up the giant tent camp on Kiev's main avenue and continue besieging official buildings until Kuchma fires Yanukovych and electoral changes become law.

On Kiev's Independence Square, thousands milled about, waving flags and posing for photographs in front of the sprawling tent camp and the throngs of people awash in orange.

Anna Ponova, who has lived in the tent camp for 10 days, said supporters would await word from Yushchenko about how long to remain encamped.

"If he wants us to stay until victory, we'll stay," she said.