With the main election just six days away, voting began at 6 a.m. at polling stations nationwide. A record 1.09 million voters have registered to cast their ballots during the two-day absentee voting period that ends at 4 p.m. Friday, the National Election Commission said.
The vote comes a day after North Korea launched a three-stage long-range rocket in defiance of international warnings and successfully put a satellite into orbit.
Navy sailors cast ballots at a polling station in the southeastern port city of Busan on Dec. 13, 2012. (Yonhap)
South Korea, the United States and other nations have condemned the launch as a disguised ballistic missile test that goes against United Nations resolutions.
The launch quickly turned the presidential race into a test of the candidates' competence concerning national security issues.
Shortly after the blast-off, ruling Saenuri Party candidate Park Geun-hye and main opposition Democratic United Party (DUP) contender Moon Jae-in denounced the North's move on their respective campaign trails and appealed to public concerns about national defense.
National security is likely to be a priority issue for more than half the registered absentee voters, or about 560,000, who are estimated to belong to the military or police force.
North Korea issues have often been used to consolidate the conservative vote in past elections.
"North Korea's missile launch can become a threat to national security in the long run," said Seo Jang-eun, a senior official of Park's election camp. "I believe service people taking part in the absentee vote will support (the conservative Park), who has a clear sense of statehood and national security."
Moon's liberal camp disagreed.
"Our soldiers who are serving on the front line will clearly judge through this situation how incompetent the current administration is in the area of national security," said Park Kwang-on, a spokesman for Moon's camp.
The opposition has lamented the government's apparent inability to detect signs of the launch in advance.
Local pollsters were also divided on the likely effect of North Korea's rocket launch.
"The general effect North Korea issues have on rallying the conservative vote could grow larger within the military and work in Park's favor," said Lee Taek-soo, the head of polling agency Realmeter.
Ji Yong-geun, the head of polling firm Global Research, offered a different view.
"Soldiers feel a lot of pressure in a confrontational situation between North and South, and when a North Korea issue arises, they immediately blame the current administration for causing the tension, which could lead them to oppose the conservatives," he said.
The absentee election comes two days after voting ended for South Koreans living overseas.
Of an estimated total of 2.23 million overseas South Koreans who are eligible to vote, 158,235 or 7.1 percent cast ballots at polling stations set up at 164 South Korean diplomatic missions in 110 countries, the election watchdog said.
The turnout over the six-day voting period was 71.2 percent of the total number of overseas South Koreans who had registered to vote.
It was the first time South Koreans living abroad took part in the country's presidential election following a revision to the election law in 2009.