Official figures showed high turnout of about 65 percent by 2 pm (0500 GMT), after record early voting, following a campaign dominated by mud-slinging between the two front runners, the incumbent Democratic Party's Lee Jae-myung and the opposition conservative People Power Party's Yoon Suk-yeol.

The pair, both so unpopular local media have branded it the "election of the unfavourables", have been neck and neck in the polls for months. Some 90 percent of the electorate supports one or the other.

The choices of young swing voters will probably prove decisive, analysts said, adding the demographic's top concerns were skyrocketing house prices in the capital Seoul, social inequality and stubborn youth unemployment.

"I'm really worried about housing prices in Seoul and I hope the new president will focus on making people's lives easier and better," Park Ki-tae, 38, told AFP after casting his ballot.

Both leading candidates have promised to build millions of new homes, although the left-leaning Lee relies more on public housing and the conservative Yoon on market-led solutions to the crisis.

South Korean politics is famously adversarial. Presidents serve a single term of five years and every living former leader has been jailed for corruption after leaving office.

The two parties are ideologically poles apart, and observers say the key question is whether voters will kick out incumbent Moon Jae-in's dovish liberals and usher in a new hawkish, fiscally conservative regime under Yoon.

"Young voters are not loyal to any particular political party and thus can't be defined by liberal-conservative ideology," said Shin Yul, political science professor at Myongji University.

"Turnouts and choices by those in their 20s will have a significant bearing on the outcome."

Voters, wearing masks and using hand sanitiser after the country recorded a record 342,446 new Covid-19 cases Wednesday amid an Omicron spike, lined up to cast their ballots at polling stations.

"What the country needs right now is change," 71-year-old Hong Sung-cheon told AFP at a polling station in southern Seoul.

Polling booths opened at 6 am will shut at 6 pm. For 90 minutes after closing, Covid-positive voters will be allowed to cast their ballots.

More than a million people were isolating at home after testing positive, health authorities said. The country amended its electoral laws last month to ensure they would be able to vote.

In a two-day early voting exercise last week, a record-breaking 37 percent of the 44 million people eligible cast their ballots -- the highest number since the system was introduced in 2013.

- North Korea -

The new president will also have to confront an increasingly assertive North Korea, which has embarked on a record-breaking blitz of weapons tests this year including a launch just days before the election.

On Tuesday, a North Korean patrol boat briefly crossed the de facto maritime border, prompting the South Korean Navy to fire warning shots. Pyongyang also tested what Seoul called a ballistic missile Saturday.

The gaffe-prone opposition candidate Yoon is more hawkish on North Korea, and has threatened a pre-emptive strike if necessary.

The former top prosecutor has also promised to abolish the gender equality ministry, saying South Korean women do not suffer from "systemic gender discrimination", despite evidence to the contrary.

"The widespread support Yoon enjoys from young men is frankly absolutely terrifying from a woman's point of view," academic and female voter Keung Yoon Bae told AFP.

Yoon's rival Lee, a former child factory worker turned politician, has offered a slew of fresh policies from a universal basic income to free school uniforms -- but his campaign has been marred by scandal.

The 57-year-old is under pressure over a controversial land development deal in which private investors profited from a state-funded project on Lee's watch as mayor of the city of Seongnam.

He was also forced to start his campaign by apologising for a profanity-laden phone call with his family involving disputes with his late brother and mother.

The winner of the election will formally succeed Moon in May. The incumbent remains popular, despite not achieving a promised peace deal with North Korea.