08:38 GMT: Turnout could prove critical. Park is popular among the older voters who are also the most dependable. Moon's camp has been out trying to encourage the younger demographic to get out and vote. This apparent generational divide has been illustrated in some interesting ways.
My colleague Lim Chang-Won witnessed a stand-up political row between one mother and grown-up daughter who perhaps unwisely decided to vote together. Eventually they cast their votes in stony silence and went their separate ways.
Another colleague, Jung Ha-Won spoke to one 61-year-old retiree, Jung Yun-Kang, who was quite open about his choice.
"Lady, look how old I am. My choice is obvious," the ex-banker said. "I've long ago reached an age when you become politically conservative and seek stability over everything."
08:33 GMT: The candidates have being doing their best to encourage people to come out to vote.
"It's freezing cold, but I plead with the people to come out and vote to open a new era for this country," Park, wrapped up in a long coat and red scarf, said as she cast her ballot in Seoul.
"This election is about our livelihoods, economic democracy, welfare and peace on the Korean peninsula," Moon said as he voted in the southern city of Busan.
08:32 GMT: With just under 30 minutes until the polls close, voter turnout appears to have been strong despite the bitterly cold weather. Long queues formed at many polling stations with voters standing patiently in the snow and slush for up to 40 minutes before getting in to vote.
As of 5:00 pm (0800GMT) the turnout had reached 70.1 percent, well above the 63.0 percent in the 2007 election. Yonhap is forecasting final turnout will be even higher.
Welcome to AFP's Live Report on South Korea's presidential election that could result in Asia's fourth-largest economy selecting its first female leader.
The ballot is a straight fight between Park Geun-Hye, the conservative daughter of assassinated dictator Park Chung-Hee, and her liberal rival Moon Jae-In, the son of North Korean refugees.
Opinion polls indicate the result could go either way, but if Park does win she will be making history by becoming the first female president of a still male-dominated nation, and the first to be related to a former leader.