Yushchenko, buoyed by the, had pushed for a quick passage of the changes, saying it's essential for having fair balloting in his Dec. 26 rerun with Prime Minister Viktor Yanukovych.
Yushchenko has urged hundreds of thousands of his supporters to continue their 13-day-old protest in Kiev's main square until the changes are passed.
But a planned marathon weekend session of parliament abruptly adjourned for 10 days without approving the package of bills, which would amend election laws and reshuffle the central election commission, which the opposition wants changed after the commission approved fraud-ridden results from the first run-off vote on Nov. 21.
The adjournment came after a compromise deal among lawmakers collapsed. Communists, socialists and pro-government factions in the parliament agreed to back the electoral changes in exchange of the opposition's support for a constitutional reform bill that would trim the president's powers and give some of them to parliament.
But Yushchenko's supporters - who resist the proposed changes in the president's powers - suggested that the parliament vote on electoral amendments now and consider the constitutional reform after the new run-off, set for Dec. 26.
Their foes cried foul and blocked the passage of the electoral changes. "No one is going to win from reneging on the agreement," said Socialist leader Oleksandr Moroz.
The opposition, in turn, accused the government of retaliating for its loss in the Supreme Court the day before. Pro-Yushchenko lawmaker Ihor Ostash called Saturday's developments in parliament "an attempt at revenge ... a blue and white revolution in parliament," referring to Yanukovych's campaign colors.
The Supreme Court's ruling invalidated the Nov. 21 vote - which the opposition claimed was rigged in favor of the prime minister - and ordered the new run-off between Yushchenko and Yanukovych, rejecting the government's attempts to call an entire new election.
Ukraine's Central Election Commission acted on the court decision Saturday and formally set the date for Dec. 26.
Yanukovych's camp said Saturday that the prime minister had decided to participate in the runoff. "There's no other way but to participate and win," Yanukovych's spokeswoman Anna Herman told journalists.
Meanwhile, thousands of opposition supporters were massed outside the parliament building, breaking into 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.
Tens of thousands of Yushchenko supporters flooded Kiev's Independence Square on Saturday, posing for photographs in front of the sprawling tent camp that the opposition has maintained for nearly two weeks, with crowds of protesters decked out in orange, their campaign color.
Anna Ponova - who confessed that after 10 days in the camp the orange scarf on her head was primarily to cover her dirty hair - said that they would await the word from Yushchenko about how long to remain encamped. "If he wants us to stay until victory, we'll stay," she said.
Yushchenko told his supporters Friday: "We have proven that we are a nation that could defend our choice." He and his aides said the protest should continue until the opposition-drafted electoral amendments become law.
Ukraine's outgoing President Leonid Kuchma accused the opposition Saturday of reneging on compromise agreements and called for quick foreign mediation of his nation's post-election political crisis.
Kuchma made the claim in a telephone conversation with Dutch Prime Minister Jan Peter Balkenende, whose country holds the rotating presidency of the European Union.
"The opposition isn't fulfilling practically any of the agreements reached at a round table that involved European politicians," Kuchma's office quoted him as saying. "That exacerbates the situation in the country."
Kuchma called for "an immediate session of the round table to discuss the fulfillment of earlier agreements."
European Union foreign policy chief Javier Solana, Polish President Aleksander Kwasniewski and other European mediators sponsored two rounds of talks over the past week with Kuchma, Yushchenko and Yanukovych.
Kwasniewski, and Jan Kubis, the chief of the Organization for the Security and Cooperation in Europe, due to arrive Saturday for what was expected to be more talks.
The Supreme Court found the runoff was distorted by widespread vote abuses in pro-Yanukovych regions. It concluded that "the violation of the principles of the election law ... make it unable to determine the voters' will."
The court's decision was a sharp rebuke to Kuchma, who had anointed Yanukovych as his successor, and a slap to Russian President Vladimir Putin, who had strongly supported Yanukovych, fearing Ukraine would tilt further to the West under Yushchenko.
In Yanukovych's stronghold in eastern Ukraine, his backers expressed anger at the court decision. The Donetsk region has already set a January referendum seeking a measure of autonomy, and Yanukovych's representative in court, Stepan Havrysh, warned that Friday's verdict could foment unrest in the east.
In the eastern city of Kharkiv, where regional officials gathered Saturday to discuss the situation, about 12,000 Yushchenko supporters demonstrated, shouting "We won't be divided!"
They outnumbered the quieter gathering of about 8,000 pro-Yanukovych backers, wearing his blue-and-white campaign colors.
Kuchma did not comment on the court's verdict, and Putin, on a state visit to India, also did not comment. A day earlier, Putin had ridiculed Yushchenko's push for a rerun.