Yushchenko did not say who was plotting against Sunday's court-ordered vote, but told supporters at a rally in Kiev that he was "calling on your courage to defend the results of the election."
"The vote on Dec. 26 will not be an easy political walk," Yushchenko said in freezing temperatures on Independence Square to mark one month since the beginning of the "orange revolution" protests. "There are some forces preparing to disrupt and they are preparing brigades, groups who are readying to come to Kiev."
"We will come on this square, this stage, after the vote on Dec. 26, and will stay until our victory is celebrated," he said.
The call echoed an appeal he made after the Nov. 21 runoff that his rival Prime Minister Viktor Yanukovych won until the Supreme Court annulled the vote, citing fraud. It ordered a new vote for Sunday.
For nearly three weeks, Independence Square was the scene of protests that paralyzed the government in this former Soviet republic. Protesters set up a sprawling tent camp on the tree-lined main street, bringing central Kiev to a halt.
Fears of violence have been high ahead of the new balloting, with rumors swirling that pro-Yanukovych supporters are being given weapons and poised to head to Kiev after the vote. Yanukovych's campaign staff has repeatedly denied the allegations.
But Yanukovych has openly warned that even if Yushchenko wins the new runoff, he will never be considered president of all of Ukraine. The heated campaign has divided the country between the pro-Yanukovych's industrial, Russian-speaking east, and the west and center where Yushchenko draws his support.
Some eastern regions have raised the possibility of pursuing autonomy if Yushchenko wins, but most of those plans appear to have dissipated in recent weeks.
"I would suggest that there is no ground to relax given ... the clear statements by some regional authorities which support Mr. Yanukovych," said Leonid Polyakov, a Kiev-based political analyst. "The issue of separatism, which potentially can include violence, could still remain on the agenda until the voting is finished."
As Yushchenko spoke, the crowd wearing orange armbands, waved orange banners chanted his name. A stage with orange Christmas trees in the square was lit up, and the rap song that became synonymous with the earlier protests again blasted from giant loudspeakers. A 23-foot-high balloon resembling a giant orange was tethered to the ground on one side of the square.
"Those were really revolutionary days — so much work to change a hated regime has never been done before," said Yushchenko, wearing a trademark orange scarf. "This history lesson will never be forgotten."
Referring to the revote, Yushchenko said: "The doors have been opened. The only thing left for us is to step over the threshold."
Yushchenko, relying on the protests and the outcry over the fraudulent vote, has found himself facing a weakened and increasingly isolated opponent. Yanukovych has been abandoned by outgoing President Leonid Kuchma and even appears to be losing the support of the Kremlin.
Yushchenko told the crowd that they changed Ukraine "peacefully, beautifully, elegantly and without any drops of blood."
"You should applaud yourselves because in this difficult period, we didn't stay in our apartments ... but came to the square," he said.
Bundled in a fur hat and coat, Lyudmila Kashchenko, 45, said: "I am tired of the revolution but I am more tired of our authorities."
Earlier Wednesday, Yushchenko pledged to make Moscow his first official destination if he is elected, a statement that came a day after Russian President Vladimir Putin promised to work with the new president.
But Yushchenko also insisted that attempts to make Russian the country's second official language had become a political issue and required extensive discussion.
The language issue has become key. Almost 70 percent of the population uses Russian on a daily basis, but Ukrainian predominates in the more nationalistic west.
Yanukovych wants Russian given official status alongside Ukrainian, which is constitutionally protected as the language of government, the police and military, universities and most schools.
Russian-language schools are also widespread and most children are bilingual. But Yanukovych supporters fear a Yushchenko presidency would lead to discrimination against Russian speakers, as has happened in other former Soviet republics.
Yushchenko has repeatedly said he would defend Ukrainians' right to speak whatever language they want.
By Mara D. Bellaby