The Constitutional Court ruling poses a last-minute logistical challenge to election officials and could provide grounds for a protracted dispute over the results of the vote -- a repeat of a November vote that was thrown out because of fraud.
The ruling came as Yushchenko and Prime Minister Viktor Yanukovych took a legally required day off from campaigning before Sunday's vote, and some 12,000 international observers -- the largest election monitoring mission ever launched -- fanned out across the country.
Sunday's vote marks the culmination of a month of upheaval in Ukraine, marked by huge protests in the streets of Kiev by Yushchenko supporters; a Supreme Court ruling that voided Yanukovych's victory in the Nov. 21 vote; tension between Russia -- which backs Yanukovych -- and the West, and revelations that Yushchenko, a pro-Western reformer, was poisoned by dioxin.
Saturday's court decision brought a new twist in the final hours before polls open. The court ruled that amendments allowing people with only certain disabilities to vote at home were unconstitutional, and it ordered that all who were unable to reach polling stations because of a disability or ill health be allowed to vote at home.
Saturday's ruling could benefit Yanukovych, who pushed for the restrictions to be lifted, saying they would deprive millions of their right to vote.
However, it could also throw an unexpected monkey wrench into his campaign team's announced plans to help disabled voters reach polling stations. They are considered a key source of backing of Yanukovych because the prime minister raised pensions during his two years in office.
The Central Election Commission was required to implement the ruling -- but it had less than 24 hours to do so, registering would-be voters and mobilizing workers to bring ballot-boxes to their homes.
"We will fulfill the decision of the Constitutional Court," said commission chief Yaroslav Davydovych. "We don't have another alternative. The vote must be held."
Yushchenko supporters had pointed to home voting as one of the tools allegedly used to commit widespread fraud in the Nov. 21 run-off between Yanukovych and Yushchenko.
Marina Stavnichuk, the deputy head of the Central Election Commission, told Associated Press Television News that the court's ruling "will remove doubt as to the legitimacy of the rerun."
The ruling does not affect other newly adopted restrictions on absentee balloting, which the opposition and Western observers said was a main vehicle for fraud.
Nestor Shufrych, a lawmaker and Yanukovych ally, said the court's ruling would affect about 3 million people, but that number could not be independently confirmed. He said Ukrainians who qualify had until 8 p.m. Saturday to notify their local election precinct that they want to voted at home.
However, it appeared unlikely that the cash-strapped Ukrainian government would be able to quickly solve the logistical problems -- and that could become a basis for legal challenges to the election results.
Shufrych said that thousands who applied for home voting were refused Saturday "because polling stations and regional election commissions did not receive instructions from the Central Election Commission." Davydovych said the commission sent out its instructions after 3:30 p.m., when it formally accepted the court's ruling.
Markian Bilinskyi, an analyst with the Kiev-based U.S.-Ukraine Foundation, said the ruling could "open a window for a substantial number of appeals."
"Depending on the margin between the two candidates, I think it gives grounds for Yanukovych's people to question the legitimacy of the vote," Bilinskyi said.
Yushchenko is considered the front-runner, with most polls showing him easily defeating his rival, who was severely weakened by the court's annulment of his victory and massive opposition protests.
Parliament passed the electoral changes this month as part of an internationally negotiated package deal that included political reforms initiated by allies of outgoing President Leonid Kuchma. The court ruling didn't affect other parts of the deal.
Police have pledged to maintain law and order during the rerun, and election officials expressed confidence that there would be no repeat of the rampant fraud that marred the November round.
"We hope that everybody has learned their lesson from the result of the previous round," said Stavnichuk. "There will hardly be anyone in Ukraine willing to falsify the elections."
At a Kiev Mass, Roman Catholic Cardinal Marian Yavorskyi and Greek Catholic Cardinal Lubomyr Husar thanked "the Lord for bestowing wisdom on the people of Ukraine and preventing an escalation of the conflict" after last month's runoff. The two urged their congregations to come to the polls "and cast ballots for a candidate they trust," the Interfax news agency reported.