* Comedian plays fictitious president in TV series
* Appeals to voters fed up with corruption, status quo
* Poroshenko paints Zelenskiy as a clownish populist
* Graphic https://tmsnrt.rs/2EEQ22R (Updates with start of voting, adds graphic, details)
By Matthias Williams
KIEV, April 21 (Reuters) - Ukrainians began voting on Sunday in an election expected to thrust a comedian with no prior political experience into the presidency of a country at war and hungry for change.
At stake is the leadership of a country on the frontline of the West's standoff with Russia following the 2014 Maidan street protests and the annexation of Crimea.
Surveys make Volodymyr Zelenskiy, who plays a fictitious president in a TV series, the overwhelming favourite to defeat incumbent Petro Poroshenko, whose popularity has been dragged down by patchy efforts to tackle corruption and sliding living standards.
Both men - who traded insults and accusations in a rowdy debate in a soccer stadium in Kiev on Friday - have pledged to keep Ukraine on a pro-Western course.
But a victory for Zelenskiy in Sunday's second-round runoff would nonetheless be a dramatic departure in a country where previous presidential elections since independence were won by experienced politicians including three former prime ministers.
Investors are seeking reassurances that whoever wins will accelerate reforms needed to attract foreign investment and keep the country in an International Monetary Fund programme that has supported Ukraine through war, recession and a currency plunge.
Zelenskiy's unorthodox campaign relied heavily on quirky social media posts and comedy gigs instead of traditional rallies and leafletting.
He has also promised to fight corruption, a message that has resonated with Ukrainians who are fed up with politics as usual in a country of 42 million people that remains one of Europe's poorest nearly three decades after winning independence from the Soviet Union.
An opinion poll by the KIIS research firm on Tuesday showed Zelenskiy with 72 percent of the vote and Poroshenko with 25 percent. Last week a different survey put them on 61 percent and 24 percent respectively.
NO CONFIDENCE IN GOVERNMENT
Zelenskiy's rise comes at a time of a political upheaval in many parts of the world, from Brexit to the election of U.S. President Donald Trump, the 5-Star Movement in Italy - also inspired by a comedian - and the rise of the far right there, in France and in Spain.
Just 9 percent of Ukrainians have confidence in their national government, the lowest of any electorate in the world, according to a Gallup poll published in March.
"I think the top election issue is frustration with the status quo," said Mary O'Hagan, Ukraine Resident Senior Director of the National Democratic Institute (NDI).
Poroshenko was elected amid high hopes for change after the Maidan protests. O'Hagan says he inherited a difficult situation in 2014 and implemented many reforms but has not convinced voters that he is serious about tackling corruption.
"I think it is fair to say that public opinion has not regarded the current set-up as a sufficient step forward from what there was before, to justify the many sacrifices that people have made following the revolution, in terms of living standards, security, loss of life, displacement," she said.
Zelenskiy remains something of an unknown quantity and faces scrutiny over his ties to a powerful oligarch who would like to see Poroshenko out of power.
Poroshenko has sought to portray his opponent as a buffoonish populist whose incompetence would leave Ukraine vulnerable to Russia. Ukrainian troops have battled Kremlin-backed separatist fighters since 2014 in a conflict in the eastern Donbass region that has killed 13,000 people despite a notional ceasefire.
Poroshenko secured visa-free travel for Ukrainians to European Union countries. He implemented some reforms, helped establish a national Orthodox Church independent from Moscow, and successfully lobbied the West to keep sanctions on Russia.
But critics say the pace of change has not matched the expectations of the protest movement. Poroshenko also did not keep to a pledge to end the war in the east within weeks.
"I'm just an ordinary person who has come to break the system. I'm the result of your mistakes and promises," Zelenskiy said during the stadium debate.
Poroshenko has described his opponent as "giggling, inexperienced, weak, ideologically amorphous". But he has struck a contrite tone since the first round of the election, apologising for mistakes and promising to be more transparent. (Writing by Matthias Williams)
Our Standards: The Thomson Reuters Trust Principles.