Supporters of opposition leader Viktor Yushchenko (search) kept up their siege of government buildings Sunday, determined to push through election law changes designed to ensure a fair vote in Ukraine's new presidential runoff even after pro-government lawmakers blocked the amendments.

Yushchenko's fiery ally, lawmaker Yuliya Tymoshenko, warned that the opposition must not assume the repeat vote will be free and fair. But she insisted Yushchenko will prevail against Prime Minister Viktor Yanukovych (search), who is backed by outgoing President Leonid Kuchma and Russia's President Vladimir Putin (search).

"We are confident of a colossal victory by Yushchenko," she told journalists, adding that fewer officials are now willing to falsify the vote. "Something has changed, thanks to the people," Tymoshenko said.

Thousands of orange-clad protesters — many grimy after living for two weeks in the opposition's sprawling tent camp on a Kiev main street — have vowed to remain until the election laws are passed. Protesters on Sunday also filled out questionnaires in the camps to sign up as polling station monitors.

As a motorcade of five police buses and several patrol cars entered downtown Kiev, the protesters scrambled to warn their comrades at the barricades. After a brief standoff, the police convoy was allowed to proceed.

"This is just a regular rotation and I am urging you to let us do our job, as you do yours," a tired-looking police captain told activists.

In Moscow, rival activists held back-to-back rallies outside the Russian Foreign Ministry on Sunday. About 100 people showed up for a rally organized by the pro-democracy Union of Right Forces to protest the Kremlin's attempts to influence the situation in Ukraine, and about the same number, led by hard-line communist Viktor Anpilov, came to demonstrate against Yushchenko, whom they consider a Western lackey.

Campaigning officially kicked off Sunday — three weeks ahead of the vote, ordered by Ukraine's Supreme Court in a landmark decision to toss out the results of the Nov. 21 runoff. Yushchenko on Saturday urged international observers to return in full force despite the Christmas holiday to help Ukraine hold a clean election on Dec. 26.

"That will be the day that will determine Ukraine's fate for decades and centuries ahead," he said.

Yushchenko also said that foreign leaders should refrain from backing a candidate.

"The election of the president of Ukraine is exclusively an internal issue for 47 million of Ukrainians," Yushchenko told British Broadcasting Corp. TV. "I wouldn't recommend to anyone in the international community to try to lend such a support to any one candidate."

Buoyed by a landmark Supreme Court ruling tossing out Yanukovych's victory in the Nov. 21 runoff vote as fraudulent, Yushchenko urged international observers to return in full force despite the Christmas holiday to help Ukraine hold a clean election on Dec. 26.

The Organization for Security and Cooperation in Europe will again deploy a full-fledged mission for the rerun, Chairman Solomon Passy pledged. More than 1,000 monitors are planning to travel to Ukraine from Canada, home to many people of Ukrainian origin, the Ukrainian Canadian Congress said.

As the tent camp, bedecked in the opposition's orange campaign color, came to life on a cold Sunday morning, a lone man stood nearby waving a white and blue flag — Yanukovych's campaign colors.

"I am here to remind them that there are others in Ukraine who do not share their beliefs," said Hryhory, a middle-aged worker from Pereyslav-Khmelnitsky, not far from the capital. He refused to give his last name.

Parliament has adjourned until Dec. 14 without passing opposition-backed legislation that would amend the election laws and reshuffle the Central Election Commission, which Yushchenko's supporters accused of covering up official fraud.

"This parliamentary break will cause the situation to further deteriorate and also increase resistance," pro-Yushchenko lawmaker Yuriy Kluichkovsky said, adding that opposition lawmakers were trying to negotiate to bring parliament back earlier.

Communists, socialists and pro-government factions in parliament refused to back electoral changes, saying pro-Yushchenko lawmakers backed out of a deal to pass constitutional reforms to transfer some of the president's powers to parliament.

Yushchenko accused his government foes of trying to trim presidential powers. He also said President Leonid Kuchma, who anointed Yanukovych as his preferred successor, was blocking changes in the electoral laws.

But the dispute showed the trouble that Yushchenko still faces from Ukraine's fractious parliament and called into question his alliance with Oleksandr Moroz, the Socialist Party leader. Moroz's support is seen as important in helping Yushchenko win votes in eastern Ukraine and urban areas.