South Korean media are projecting that conservative candidate Park Geun-hye has won Wednesday's election, which would make her the first ever female president in the country's history.
With partial returns counted, at least three major South Korean media outlets are projecting a narrow victory for Park, the five-term lawmaker and 60-year-old daughter of South Korean dictator Park Chung-hee.
She was locked in a tight race with her liberal rival, Moon Jae-in, a former human rights lawyer who was once jailed under the administration of Park's father.
Despite frigid winter weather, turnout was strong across the country, surpassing that of the previous two presidential elections. Officials were not expected to count all the votes and deliver an official result until late Wednesday.
Earlier, Park told reporters she was optimistic as she cast her vote in the capital, Seoul.
"Although the weather is cold, I hope everyone participates in voting and opens the new era that people have been waiting for. I will await your decision with a humble mind. I believe the wise South Korean people will lead the way to the Republic of Korea's new era."
Moon, a center-left candidate from the opposition Democratic United Party, cast his vote in the southeastern port city of Busan.
"I have tried my best. I have done all I could and everything is at God's disposal. The only way for our citizens to change the world is to vote."
Both candidates have promised to fix the widening income gap that has expanded under President Lee Myung-bak, who is stepping down at the end of his five-year term.
The 60-year-old Park has pledged dialogue with North Korea, but has taken a tougher stance on Pyongyang's nuclear and missile programs. Moon would go a step further, resuming aid without preconditions to the impoverished country.
North Korean defector Choi Young-hee, who escaped from Pyongyang in 2004, is casting ballots in her second presidential election in the South. She says she is disappointed in the hard-line stance President Lee's government has taken toward the North.
"Our earnest wish is unification. I have left my family in the North, survived a life-or-death crisis, and I cannot write a letter to them or see them. When unification is achieved I hope that we get to meet our family and live a happy life. I believe the next president will make our earnest wish a reality."
Moon is a former chief of staff to Mr. Lee's predecessor, late President Roh Moo-hyun, who advocated the so-called "Sunshine Policy" of better relations with the North.
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.