Ukraine votes Sunday in a presidential election aimed at calming a deadly crisis that has threatened the ex-Soviet country\’s very survival and plunged East-West relations to a post-Cold War low.
Voting in the eastern regions of Donetsk and Luhansk has been seriously disrupted.
There were no polling stations open in Donetsk city, and across the region only seven out of 12 district electoral commissions were operating.
Ukraine has mobilised more than 55,000 police and 20,000 volunteers to ensure security for the vote, being overseen by 1,200 international monitors.
Polling stations will remain open until 20:00 (17:00 GMT), with definitive results expected on Monday.
The country fears a new wave of violence by pro-Russian separatists who started their uprising seven weeks ago and declared independence of Luhansk and Donetsk, prompting clashes with security services and demonstrators for united Ukraine.
There are 18 candidates on the ballot, which is widely seen as a crucial moment to unite the country.
Former Prime Minister Yulia Tymoshenko is lagging behind Mr Poroshenko in opinion polls.
If no candidate wins more than 50% of the vote, a second round will take place in June.
The Organization for Security and Co-operation in Europe deployed 1,000 observers but withdrew its teams from Donetsk over fears for their security.
Sunday\’s vote comes with tensions running high after a bloody upsurge in fighting, with two Western photographers injured while covering the disturbances in the country\’s separatist east.
Prime Minister Arseniy Yatsenyuk issued an appeal for people to turn out to "defend Ukraine", which has been in deep crisis since street protests forced out the Kremlin-backed regime in February.
"This will be the expression of the will of Ukrainians from the west, east, north and south," he said.
At least 150 people died in the unrest.
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.
Russian president Vladimir Putin says he will respect the choice of the Ukrainian people in Sunday\’s presidential vote but says ousted ex-leader Viktor Yanukovych technically remains in power.
Speaking at an investment forum in St Petersburg on Friday, Putin said Russia would "respect the choice of the Ukrainian people" and would work with the new leadership.
The presidential elections were called after the last elected President, Viktor Yanukovych, was deposed in February amid mass protests against his pro-Russian policies.
He said he was upbeat on the prospects for resolving the crisis in Ukraine and that doing so would help improve relations with the United States.
"I\’m an optimist. I am not losing faith that the situation in Ukraine will at some point become normal and we will find the inner strength to normalise relations (with the United States)," the Reuters news agency reported.
He also said that Russia did not want to isolate itself from the rest of the world and that it wanted to work with the United States on many projects.
"We are not planning any self-isolation," Putin said. "We hope that common sense … will prompt our European and US partners to work with Russia,"
Yet earlier, Putin had blamed the West for both encouraging a "coup" in Ukraine when the nation\’s pro-Russian president was chased from power and for plunging the country into what he described as "chaos and a full-scale civil war."
In a live televised address from Kiev, acting Ukrainian President Oleksandr Turchynov, who is not running in Sunday\’s presidential vote amid a surge a attacks by pro-Russian separatists in the country\’s east, emphasised the importance of choosing a new leader.
"Today, we are building a new European country the foundation of which was laid by millions of Ukrainians who proved that they are capable of defending their own choice and their country," Turchynov said, the AP news agency reported.
Turchynov said on Thursday that 14 troops had been killed in two overnight attacks by rebel forces in the separatist east of the country.
Ukrainian security forces and the pro-Moscow separatists have clashed repeatedly in recent weeks in eastern Ukraine, where the breakdown of security has rattled the pro-Western interim government in Kiev.
Russia denies the legitimacy of the current Kiev government, which took over after mass street protests toppled Moscow-backed president Viktor Yanukovich in February. He fled to Russia and in March Russia seized Ukraine\’s Black Sea peninsula of Crimea.
Ukraine\’s new president will be faced with a challenge of finding a common ground with Russia, maintaining close ties with the West without antagonising Moscow, and saving the country\’s collapsing economy.