KIEV, Ukraine (AFP) — Ukraine votes Sunday in a presidential election aimed at calming a deadly crisis that has threatened the ex-Soviet country’s very survival, with a reported cyber attack on the vote-counting system adding to concerns.

A bloody pro-Russia rebellion is set to scuttle voting in the east, a region shaken by the thunder of military tanks and grenade launchers since mid-April.

Prime Minister Arseniy Yatsenyuk issued an appeal for the country’s 36 million voters to turn out in force on Sunday to “defend Ukraine” in the most important election since independence in 1991.

“This will be the expression of the will of Ukrainians from the west, east, north and south,” he said Saturday.

But the insurgents have threatened to block voting in their strongholds in the rust belt on the Russian border “by force if necessary”, ratcheting up the tension in a crisis that has plunged East-West relations to a post-Cold War low.

The West regards the vote as a crucial step in preventing Ukraine from disintegrating further after Russia seized Crimea in March, and has warned Moscow of further sanctions if it disrupts polling.

President Vladimir Putin — still authorized by parliament to invade Ukraine if necessary to “protect” ethnic Russians — appeared to make a big concession Friday by saying he was ready to work with the new Kiev team.

“We understand that the people of Ukraine want their country to emerge from this crisis. We will treat their choice with respect,” he said.

Russia also says it has started withdrawing from Ukraine’s border around 40,000 soldiers and dozens of tank battalions that had been ready to advance at a moment’s notice.

The packed field of candidates features clear front-runner Petro Poroshenko — a chocolate baron and political veteran who sees Ukraine’s future anchored to Europe — and 17 far less popular hopefuls that include ex-premier Yulia Tymoshenko.

But before voting got under way Ukraine’s Interior Minister Arsen Avakov said that the election computer system had been the victim of a cyber attack and that counting would have to be done manually.

“Somebody really broke into the system, erased all data and disrupted the communication with constituency electoral commissions,” Avakov said on his personal website.

“This is not a catastrophe. We will count manually. The results will be more accurate,” the Interfax-Ukraine news agency quoted the minister as adding.

The election should give the new president and his government a stamp of legitimacy after pro-EU protests forced out Kremlin-backed president Viktor Yanukovych in February, setting off the chain of once-unimaginable events that now threaten the country’s unity.

“This election will be decisive for Ukraine,” said Kiev’s Institute of Global Strategies director Vadym Karasyov.

“This is the first time Ukraine is voting not as a post-Soviet country but an independent one.”

However final opinion polls show self-made 48-year-old billionaire Poroshenko falling just short of the 50-percent threshold needed to avoid a second round on June 15, and three weeks of further political uncertainty.

Corruption-stained Orange Revolution co-leader Tymoshenko is battling for the runner-up spot against hawkish former defense minister Anatoliy Grytsenko — a supporter of Ukraine’s membership in NATO — but is expected to be crushed by Poroshenko in any runoff.

The new president is expected to try to repair relations with Ukraine’s former masters in Russia while pushing the nation of 46 million along a westward track.

Ukraine is also hoping that up to $27 billion (20 billion euros) in global assistance it won after the old regime’s fall may help avert threatened bankruptcy and revive growth after a contraction estimated by the IMF at about five percent this year.

Ukraine is mobilizing more than 55,000 police and 20,000 volunteers to ensure security for the vote, being overseen by 1,200 international monitors.

But the authorities acknowledge there will be problems staging polling in the steel mill and coal mine-dotted regions of Donetsk and Lugansk, where rebel leaders have already declared independent republics after hasty May 11 referendums.

Sunday’s snap ballot was called by Kiev’s interim leaders who took power after Yanukovych fled in the bloody climax of months of protests sparked by his rejection of a historic EU alliance.

The charred buildings and flower-heaped barricades still crisscrossing Kiev’s Independence Square — also the cradle of the 2004 Orange Revolution that first shook Russia’s historic hold on Ukraine — serve as testimony to the heavy trauma suffered by the young nation in those bloody winter days.

The early euphoria in Kiev and the ethnic Ukrainian west over the fall of a corruption-stained regime has given way to disillusionment and disbelief at Russia’s annexation of Crimea.

And the seven-week separatist uprising that has claimed at least 150 lives in the east leaves open the very real possibility of nearly seven million Ukrainians severing ties to Kiev.

Washington and its European allies have used the threat of sanctions against entire sectors of Russia’s economy to pressure the Kremlin into recognising the election outcome.

Putin’s inner circle and select Kremlin-linked tycoons have already been blacklisted by the United States — a measure EU nations that rely on Russian trade and energy supplies have applied to far less senior officials.

Voting begins at 0500 GMT and closes at 1700 GMT, with first results expected from 2100 GMT.