South Korean Presidential candidates made last minute appeals to rally support on Monday as the nation of 51 million elects a new President on Tuesday after a year of political upheaval. Latest polls have put Democratic Party's Moon Jae-in in the lead, the media reported.

The early election was triggered after President Park Geun-hye was ousted from office in March following her involvement in a massive corruption scandal.

Moon's closest rivals are conservative Liberty Korea Party candidate Hong Joon-pyo and the People's Party candidate Ahn Cheol-soo, who is also a liberal. Both are close to 20 per cent in the polls.

Moon made a final pitch to voters Monday. "Without the united power of our people, (the new administration) will struggle from its first step. We must overcome this crisis with united power," he said.

Hong also made a final plea to voters Monday to unite around him to protect conservative free market and strong on defence policies that have been in place for the last eight years, VOA reported..

"I stood up to prevent the left-wing from seizing power, and acted to protect a free Republic of Korea. We will win. Truth will beat lies. Our people will win. Justice will beat hypocrisy. The Republic of Korea will win," said the Liberty Korea Party candidate.

As part of the campaigning, Moon is to make a trip to the country's second-largest city, Busan, and return to Seoul to finish his campaigning, Yonhap News Agency reported.

Busan, a port city located 450 km southeast of Seoul, apparently holds the most swing votes for the liberal candidate as it is considered the traditional home of conservatives.

The 64-year-old former lawyer and chief of staff to late President Roh Moo-hyun, is also scheduled to make stops in Daegu, another stronghold of conservative parties, and Cheongju, a city located in the North Chungcheong province, according to party officials.

Hong Joon-pyo, 62, from the Liberty Korea Party is to campaign in Busan and Daegu before making a brief stop in Cheonan in South Chungcheong province.

The third leading contender, Ahn Cheol-soo, 55, of the centre-left People's Party is focusing last minute efforts on Daejeon and the Chungcheong provinces.

Ahn had maintained a strong second place in most polls up until the very last minute when he appeared to have been overtaken by Hong.

The last opinion poll on the presidential election published on May 3 had Ahn and Hong tied at 18.6 per cent with Moon in the lead with 42.4 per cent. 

The publication of new opinion polls has been prohibited since.

Voting will begin at 6 a.m. on Tuesday and end at 8 p.m.

According to the National Election Commission (NEC), a winner will likely emerge around 3 a.m. on Wednesday morning, while the official declaration is expected after 8 a.m.

On May 5, a record number of South Koreans participated in early voting conducted in 3,507 polling stations across the country. 

Over 42.4 million people are eligible to cast ballots on Tuesday, marking the largest number of voters in the country's history, the NEC said.

All votes cast in advance will be kept sealed with those locked overseas to be brought back for official counting following the end of Tuesday's voting, the election watchdog added.