South Korea's two leading presidential candidates are in a very tight race, with one opinion poll putting the main opposition's Yoon Suk-yeol ahead of the ruling party's Lee Jae-myung and another survey showing contrary results.

Yoon of the People Power Party (PPP) had 40.4 percent, down 4 percentage points from a week ago, while Lee of the Democratic Party (DP) posted 39.7 percent, up 1.7 percentage points from a week earlier, according to a Realmeter poll conducted on 3,090 adults between Sunday and Friday.

Their gap was within a margin of error of plus or minus 1.8 percentage points and is sharply down from 6.4 percentage points a week earlier.

But a Korea Society Opinion Institute (KSOI) survey of 1,000 people put Lee ahead with 37.6 percent against Yoon's 35.8 percent.

Their 1.8 percentage point gap is also within the margin of error of plus or minus 3.1 percentage points, with a 95 percent confidence level, and is down from 2.7 percentage points a week ago.

Ahn Cheol-soo, the presidential candidate of the minor opposition People's Party, came third with 5.6 percent, followed by Sim Sang-jeung of the minor progressive Justice Party at 2.9 percent in the Realmeter survey.

Ahn and Sim garnered 7.3 percent and 3.5 percent, respectively, in the KSOI survey.

Of the Realmeter poll respondents, 37.2 percent supported the PPP, while the DP had 32.7 percent support. The gap was narrowed to 4.5 percentage points from 6.1 percentage points a week earlier.

In the KSOI survey, the PPP registered 31.2 percent and the DP had 30.8 percent support. (Yonhap)