* Presidential election follows months of upheaval
* At least eight Ukrainians killed in overnight clashes
* Kiev accuses Moscow of undermining Ukraine's democracy
By Pavel Polityuk and Natalia Zinets
KIEV, May 22 (Reuters) - Ukraine said on Thursday its forces had rebuffed overnight attacks by armed pro-Russian separatists on an army checkpoint and a border crossing and called for an emergency session of the U.N. Security Council to discuss Moscow's role in the violence.
With tensions rising ahead of Sunday's presidential election in Ukraine, security sources said eight soldiers had been killed in fighting at the checkpoint, while some border guards were hurt when "dozens" of gunmen tried to enter Ukraine from Russia.
The election is meant to stabilise Ukraine after mass street protests toppled Moscow-backed president Viktor Yanukovich in February, but the separatists have vowed to prevent the poll going ahead in eastern towns where they have seized control.
The United States and European Union say they will impose broad sanctions on Russia, which annexed Ukraine's Crimea peninsula in March after Yanukovich's fall, if it tries to derail the election.
The pro-Western interim government in Kiev urged people across the country to take part in the election in order to "defeat" Russian President Vladimir Putin and the rebels.
Opinion polls suggest confectionary magnate Petro Poroshenko, an ally of the former president who later joined protests against him, will win the election, billed as the most important since Ukraine won independence from Moscow in 1991.
Top Ukrainian security official Andriy Parubiy told a news conference he expected more separatist violence in the coming days "because their whole concept is aimed at disrupting the presidential elections".
"I would like to appeal to all citizens of Ukraine, not only to those in the east: on Sunday ... we must all go and vote ... Going to the elections, holding the elections means defeating Putin," Parubiy told a news conference.
Security sources said that along with the eight killed, at least 18 Ukrainian security personnel were wounded in clashes with the separatists, who have declared "people's republics" in the Donetsk and Luhansk regions after referendums this month.
The main clash took place about 20 km (12 miles) south of the city of Donetsk, an industrial hub of one million people.
The defence ministry said gunmen had opened fire on an army checkpoint near the town of Volnovakha but gave no death toll.
In the Luhansk region, Ukrainian border guards repelled a cross-border incursion by dozens of separatists armed with grenade launchers and rifles, the border service said.
Ukraine's foreign ministry said a Russian helicopter had also violated Ukrainian airspace late on Wednesday.
"Russia continues to violate its international obligations and principles of international relations, hypocritically ignoring the Geneva agreement (designed to reduce tensions in Ukraine), deliberately choosing tactics to further aggravate the situation in Ukraine," the ministry said in a statement.
Interim Prime Minister Arseny Yatseniuk said Kiev was ready to submit evidence of what he called Moscow's attempts "to escalate the conflict" to an emergency session of the U.N. Security Council, of which Russia is a permanent member.
Moscow, for its part, accused Kiev on Thursday of stepping up military operations in eastern Ukraine and of failing to implement measures aimed at ending the crisis.
NATO has accused Russia of amassing tens of thousands of troops across the border from eastern Ukraine. On Thursday, Moscow announced it had moved some troops and military equipment from the Ukraine border area but NATO's top military commander said Russia's forces in the region remained "very large".
Kiev says Sunday's election cannot be held in parts of the Donetsk and Luhansk regions and says Moscow is deliberately seeking to undermine Ukrainian democracy, a charge echoed by the United States and the EU.
Russia denies the legitimacy of the current Kiev government and has asserted its own right to intervene on behalf of Russian speakers outside Russia's borders.
Election front-runner Poroshenko has urged voters to hand him victory in Sunday's first round of voting, suggesting that Ukraine's deteriorating security situation might otherwise derail the election before a second round can be held.
If no candidate wins more than 50 percent of the vote on Sunday, a second round will be held on June 15.
A poll watchdog has said it expects a turnout of at least 70 percent of voters nationwide in Sunday's election, despite the loss of Crimea and the turmoil in the east. (Writing by Gareth Jones; editing by Philippa Fletcher)
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