Thousands of supporters of South Korea's two leading presidential candidates rallied on Saturday in Seoul, days before 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.

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

Kim Moon-soo, from the conservative People Power Party that Yoon left this month, trailed behind at 35 percent.

Organisers from both camps told police they expected tens of thousands of supporters to rally in Seoul on Saturday.

In Seocho, in the south of the capital, Lee supporters gathered holding signs condemning Yoon's "insurrection".

"I believe the outcome of the presidential election is already decided," said Lee Kyung-joon, a Lee supporter.

"I came to today's rally to help condemn the forces involved in the martial law attempt."

Yoon is currently on trial for insurrection, and Kwon Oh-hyeok, one of the organisers of Saturday's rally, said a Lee victory in the June 3 vote was crucial to holding him accountable.

"Isn't the People Power Party's decision to run in the snap election -- triggered by Yoon's removal from office -- an insult and a betrayal of the people?" Kwon told rally participants.

"Fellow citizens, we must win by a landslide to deliver the justice this moment demands."

Both frontrunner Lee of the liberal Democratic Party and conservative challenger Kim have cast the race as a battle for the soul of the country.

More than a third of those eligible cast their ballots in early voting on Thursday and Friday, according to the election commission.

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

Experts say that regardless of who wins, South Korea's polarisation is likely to deepen.

If Lee wins, the conservatives "will do whatever it takes to undermine him and his government, whether their logic makes sense or not", political analyst Park Sang-byung said.

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

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. (AFP)