Outgoing President Leonid Kuchma said the vote on Sunday "will likely be the most important day in Ukraine's existence" and told his security chiefs late Thursday, "I say without exaggeration that this election round will greatly influence Ukraine's fate."
The 11th-hour push for votes by western-leaning opposition leader Viktor Yushchenko and his pro-Russian rival, Prime Minister Viktor Yanukovich, came after Russian President Vladimir Putin again attacked the West's "double standards" in Ukraine and elsewhere in the former Soviet Union.
At his annual press conference at the Kremlin, Putin said he would ask US President George Bush whether the west was purposely seeking to isolate Russia from the other states that were once part of the Soviet Union.
Ukrainian law forbids campaigning on a day before an election, leaving Yushchenko and Yanukovich with only Friday to get their message out before polling day.
Yushchenko was scheduled to hold a news conference in Kiev at 12.30pm (1030 GMT) and to make a televised speech late on Friday while Yanukovich, who delivered his own nationally televised speech on Thursday night, was due to campaign in the western city of Uzhgorod.
The two men are battling to take over the helm after 10 years of authoritarian-leaning rule by Kuchma and the outcome of their contest will determine whether the nation, sandwiched between the European Union and Russia, will turn westward or remain in Moscow's orbit.
Yushchenko, 50, tends to be more pro-western, advocating eventual membership for Ukraine in the NATO military alliance and the European Union.
Yanukovich, 54, favours Ukraine keeping closer ties to Russia.
The 'orange revolution' has roiled
Ukraine for several weeks now
Yushchenko, the hero of the "orange revolution" that roiled Ukraine and echoed throughout the world, has a comfortable 14-point lead in opinion polls over Yanukovich, whose political fortunes tumbled after he was stripped of his victory in an earlier poll due to fraud.
The two men have competed to get their message out to the voters via the media in the past few days.
"My hand will always defend your interests," Yushchenko assured residents in eastern Ukraine, where support for his opponent is strong and resentment over the "orange revolution" widespread.
"Have no doubt, be it a question of language or confession ... I would like to say firmly that your interests will form a basis for my actions," Yuschenko said in a television interview late Thursday.
Yanukovich, meanwhile, called on voters to back his vision of Ukraine instead of the "scary image" of the opposition and vowed to stay in politics.
"I am not going to leave politics," he said in a taped message. "I cannot shirk my responsibility for the country."
Sunday's election is a rerun of a 21 November vote officially won by Yanukovich but later annulled by the supreme court because of alleged widespread fraud.