SEOUL: 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.

Organisers 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.