MOSCOW -- Ukrainians vote Sunday in an early presidential election that could be a crucial step toward resolving the country's crisis, but separatists in the east are threatening to block the vote. The election -- which comes six months after the outbreak of protests that led to the president's ouster and a deepening chasm between pro-Europe and pro-Russia Ukrainians -- aims to unify the fiercely divided country or at least at discourage further polarization.

A look at the vote:

Why the early election?

After months of protests against his rule and scores of protesters killed by snipers, President Viktor Yanukovych signed an agreement with opposition leaders on Feb. 21 calling for early presidential elections by December. But he fled later in the day, eventually resurfacing in Russia, and parliament decided to hold the presidential election May 25.

Since Yanukovych's ouster, Russia has portrayed the interim government, including acting president Oleksandr Turchynov, as a junta, and annexed Crimea in March.

Moscow's animosity toward the authorities in Kyiv has fed the tensions in eastern Ukraine, where at least two regions have declared independence. If Ukraine is able to elect a president in a democratic and transparent process, that will counter Russia's argument that the government is illegitimate.

Who's on the ballot

Twenty-one candidates are running and about 35 million people are eligible to vote. Polls show billionaire candy-maker Petro Poroshenko with a commanding lead but falling short of the absolute majority needed to win in the first round. His nearest challenger is Yulia Tymoshenko, the divisive former prime minister, but her support is only 6 per cent. If no one wins in the first round, a runoff will be held June 15 -- polls indicate Poroshenko would win that contest.

Poroshenko is getting support for his pragmatism and an apparent willingness to compromise -- unusual qualities in a political landscape dominated by vehemently inflexible figures. He supports Ukraine developing closer ties with the 28-nation European Union but also says he recognizes the importance of pursuing good relations with Russia.

Threats against voting in the east

Much of eastern Ukraine is gripped by unrest. Pro-Russia insurgents are clashing with Ukrainian forces there and have declared independence for the Donetsk and Luhansk regions -- an area that encompasses 6.6 million people. Rebel leaders say they will do all they can to prevent the vote from taking place.

Government officials admit that voting won't be possible in some eastern areas; even if polling stations are functioning, residents intimidated by threats and gunmen may not risk voting.

The validity of an election that is nominally national but can't be conducted in some parts of the country is a delicate issue. The Organization for Security and Cooperation in Europe is sending a large observer mission and its report should have significant influence, but the missions do not make outright assessments of an election's validity.

Russian President Vladimir Putin said Friday that Russia would recognize the results of the vote and work with Ukraine's new leader, but voiced hope that a government military offensive against separatists in the east would end.

The long road ahead

Whoever wins faces daunting challenges, from resolving Ukraine's dire financial straits to unifying its divided electorate and pushing new laws through a fractious parliament.

Six months of heated crisis have galvanized extremist sentiments in both camps -- those who regard Russia as their protector and the nationalists who despise Russia's influence. Thursday's attack that killed 16 Ukrainian soldiers appears to show that the eastern separatists are prepared for significant violence. Pro-Europe protesters, meanwhile, are still camped out in Kyiv's main square and the nationalist Svoboda party has a substantial presence in parliament.

The president will also have to struggle with Ukraine's economy, hobbled by widespread corruption and a $3.5 billion debt to Russia for natural gas imports. Yanukovych's regime is widely believed to have siphoned off billions more for officials' personal gain. The country got a temporary boost from a $17 IMF loan package this year, but will need to make painful economic reforms.

Russia and the insurgents have also demanded more power for Ukraine's regions. The parliament recently passed a vague memorandum on the topic, but the new president faces the challenge trying to put those proposals into action.