Former South Korean opposition leader Moon Jae-in on Thursday continued to lead in the presidential poll to become the nation's new leader.

According to a Realmeter survey released on Thursday, Moon, who led South Korea's biggest opposition Minjoo Party from 2015 to 2016, gained 37.1 per cent in approval ratings as the next leader. It was up 2.0 percentage points from the previous week, Xinhua news agency reported.

The results are based on a poll of 1,015 voters conducted on Wednesday. It has 3.1 percentage points in a margin of error.

The country entered a presidential race as the constitutional court upheld the motion to impeach President Park Geun-hye on March 10 on charges of corruption. 

A presidential election to succeed Park is set for May 9.

Prime Minister Hwang Kyo-ahn on Wednesday declared his withdrawal from the run for president but there was no dramatic change in opinion poll rankings among presidential contenders.

Before his declaration, Hwang came next to Moon in presidential polls though there had been a wide difference.

Governor of South Chungcheong province Ahn Hee-jung of the biggest opposition Minjoo Party ranked second with 16.8 per cent in approval scores, up 2.7 percentage points from a week earlier.