Supporters of South Korea's two leading presidential candidates were due to rally Saturday in Seoul, days ahead of a vote triggered by the ex-leader's disastrous declaration of martial law.

Tuesday's election caps months of political turmoil sparked by Yoon Suk Yeol's brief suspension of civilian rule in December, for which he was impeached and removed from office.

South Korea has since been led by a series of lame-duck acting presidents as its export-driven economy grapples with trade turmoil abroad and sluggish demand at home.

And both frontrunner Lee Jae-myung of the liberal Democratic Party and conservative challenger Kim Moon-soo have cast the race as a battle for the soul of the country.

Organizers from both camps have told police they expect tens of thousands of supporters to rally in Seoul's central Gwanghwamun square and the southern Seocho neighbourhood on Saturday afternoon.

Over a third of those eligible have already cast their ballots in early voting, according to Seoul's National Election Commission.

Overseas voting reached a record high, with nearly four-fifths of the 1.97 million eligible voters casting their ballots last week.

All major polls have placed liberal Lee well ahead in the presidential race, with a recent Gallup survey showing 49 percent of respondents viewed him as the best candidate.

Kim, from the ruling People Power Party that Yoon left this month, trailed behind at 35 percent.

Lawyer-turned-politician Lee has pledged to bring those behind the martial law declaration to justice and told supporters they are voting for "revolution".

And former labor minister Kim — who rose to fame when he refused to bow in apology for that bid to suspend civilian rule -- has vowed to defend the country from "extremist forces" seeking "chaos".

Whoever succeeds Yoon will have to grapple with a deepening economic downturn, one of the world's lowest birth rates and a soaring cost of living.

He will also have to navigate a mounting superpower standoff between the United States, South Korea's traditional security guarantor, and China, its largest trade partner.