Undecided voters represent up to 10 percent percent of South Korea's 40 million eligible voters, the Yonhap News Agency reported.
"The candidate who can successfully court voters who ... have not made up their minds, will likely win the race," said political analyst Kim Sung-wan.
The ruling Saenuri Party's presidential hopeful, Park Geun-hye, has has a lead over her main opposition, Moon Jae-in from the Democratic United Party, but observers say if undecided voters go for Moon, he could win.
"Park has a slight edge at present, yet Moon seems to have caught up in the past week so the actual results are hard to predict," analyst Kim Jong-bae said.
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