WARSAW (Reuters) - Poles vote for a president on Sunday in a run-off that is too close to call but already spells trouble for the ruling Civic Platform as it looks to re-election later in the year.

Originally seen as a shoo-in for a second five-year term, incumbent Bronislaw Komorowski finds himself neck and neck with his conservative challenger, Andrzej Duda, who defied opinion polls to come out on top in the May 10 first round.

Victory for 43-year-old Duda would mark the first major electoral win in almost a decade for the opposition Law and Justice party, and provide a springboard to challenge for government in the autumn.

Even if he loses, the race has turned former legal aide and European MP Duda into a national political player and sounded a warning to the ruling Civic Platform.

Eight years in power, the center-right Civic Platform has presided over rapid economic growth and rising salaries in eastern Europe's biggest economy. But despite unprecedented prosperity, many Poles feel the fruits of their labor have been unfairly spread, resulting in increased inequality, and are eager for new faces at the top.

"The current president had five years to prove himself, and now I want something new," said 22-year-old student Dominika Gaszcz.

In Poland, it is the prime minister who leads the government, but the president is head of the armed forces, has a say in foreign policy and in the passage of legislation and also controls who heads the central bank.

"Perhaps these years have been good, but only for a narrow group," Duda told supporters in his native Krakow, southern Poland, on Friday.

"Yes, it's time for change, time to end this sluggish, indolent presidency."

Komorowski, 62, has been criticized for running a lackluster campaign, jumping to life only after his loss in the first round. Since then, he has shown more energy and two opinion polls on Friday had the contenders practically level.

"The polls suggest it may be thousands of votes that will decide, perhaps even hundreds, whether Poland will drown in arguments, become an embarrassment to the world because of the views of such people," Komorowski said on Friday of Duda and the Law and Justice party that backs him.

Piotr Matys, emerging markets strategist at Rabobank, said: "Even if Duda fails to dethrone President Komorowski, I think that the Civic Platform may not stay at the helm after the general election due later this year."