Ukraine's highest court ruled Saturday that a recent electoral reform — restricting who can vote from home — was unconstitutional, creating a massive logistical challenge on the eve of the presidential revote.

Some 12,000 international observers — the largest such mission ever launched — fanned out across Ukraine (search) on Saturday as elections officials scrambled to comply with the Constitutional Court ruling, one that could provide grounds for another protracted challenge to Sunday's vote.

"We don't have another alternative. The vote must be held," said Central Election Commission chief Yaroslav Davydovych.

The two candidates, Prime Minister Viktor Yanukovych (search) and opposition leader Viktor Yushchenko (search), were not allowed to campaign Saturday and did not comment on the ruling.

The court ruled that amendments allowing only people with certain disabilities to vote at home in Sunday's poll were unconstitutional, and ordered that all those unable to reach polling stations because of a disability or ill health be permitted to cast ballots at their homes.

Yushchenko supporters had pointed to home voting as one of the tools allegedly abused by Yanukovych supporters in the Nov. 21 presidential runoff. Citing widespread fraud, the Supreme Court invalidated the results of that vote, canceling Yanukovych's victory and ordering the revote.

Central Election Commission officials insisted they could implement the ruling before the vote, mobilizing workers to identify those voters and arrange to bring ballot boxes to their homes.

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.

Nestor Shufrych, a lawmaker and Yanukovych ally, said the court's ruling would affect about 3 million people, a number that could not be independently confirmed. He said Ukrainians who qualify had until 8 p.m. (1800 GMT) Saturday to notify their local election precinct that they want to voted at home.

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, the commission chief, said that the commission had sent out its instructions after 3:30 p.m. (1330 GMT), when it had 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.

Yanukovych enjoys strong backing from elderly and disabled Ukrainians who saw their pensions raised twice during his time as prime minister, and he had pushed for the restrictions to be lifted, saying they would deprive millions of their right to vote.

But it was unclear if the ruling would help or hurt his prospects. His campaign workers had planned to ferry many homebound elderly to the polls, and logistical concerns made it far from certain that more would get the chance to cast ballots from home under the rule changes imposed Saturday.

The ruling does not affect other newly adopted restrictions on absentee balloting, which the opposition and Western observers said became one of the main vehicles for fraud.

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.

Police have pledged to maintain law and order during the rerun.

Police were investigating an attack Saturday afternoon against Yushchenko campaign headquarters in the city of Sverdlovsk in the eastern Luhansk region, said Oleksandr Kulchytskyi, an election monitor from the opposition leader's camp. He said that a group of seven drunk men damaged several cars bearing Yushchenko's orange campaign color with bats and rocks and slightly injured a number of pro-Yushchenko monitors.

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 Marina Stavnichuk, the deputy electoral commission chief. "There will hardly be anyone in Ukraine willing to falsify the elections."