A Gallup poll released on Tuesday indicates South Korean opposition leader Lee Jae-myung is the clear frontrunner to become the country's next president, with less than a week to go until the elections.

South Koreans go to the polls next Tuesday, capping months of political turmoil triggered by ex-leader Yoon Suk-yeol's ill-fated effort to suspend civilian rule in December.

Gallup put Democratic Party leader Lee as the clear frontrunner, with 49 per cent of the respondents stating they saw him as the best candidate.

Trailing behind is conservative former labour minister Kim Moon-soo of the ruling People Power Party at 35 per cent.

In third place is Lee Jun-seok of the Reform Party – running a campaign targeting South Korean youth – with 11 per cent.

The poll, conducted by Gallup Korea over the phone, surveyed 1,004 respondents over the weekend, with a stated margin of error of 3.1 percentage points.

The poll suggested the Democratic Party leader would win handily in almost all over the country, securing the most populous regions of capital Seoul, Incheon and Gyeonggi.

Third place Lee Jun-seok has faced growing pressure from conservatives to abandon his presidential bid to stop left-leaning Lee Jae-myung from winning.

But the 40-year-old has remained firm, declaring that his name will be "clearly visible" on the ballot on election day.

Tuesday's poll indicated Lee Jae-myung would win even if he faced a unified conservative candidate.

Around 5 per cent of those polled said they had no preference or that they did not know who would be the best candidate.

The survey is one of the last to be released before next week's vote, with further polls in the run-up banned. (AFP)