Polls opened early Wednesday in South Korea's closely contested presidential election, which pits the daughter of a former military ruler against a former human rights lawyer.
Voting will end at 6 pm local time, or 0900 UTC, and results are expected later in the day.
Opinion surveys released last week showed Park Geun-hye of the conservative ruling New Frontier Party holding a slight lead over her liberal rival, Moon Jae-in of the center-left main opposition Democratic United Party.
Park is the daughter of the late South Korean dictator Park Chung-hee. If elected, she will be the country's first female president. She also would be the first South Korean head of state to be related to a former president.
Moon once was jailed for protesting against the government of Park's father.
Both candidates have made late appeals to moderates, promising to fix the widening income gap that has expanded under President Lee Myung-bak's administration.
One area of difference is North Korea. Moon pledges to hold an early summit with Pyongyang and resume aid without preconditions. Park is hesitant to make concessions until North Korea apologizes for recent military provocation.
North Korea last week fired a long-range rocket that many say was partially intended to influence the vote in South Korea. But observers said it was unclear whether the satellite launch will have any effect on the outcome.
Park's father, who ruled the country for 18 years, is both admired for dragging the country out of poverty and reviled for his suppression of dissent. He was assassinated in 1979.