SEOUL - The most powerful presidential contender in South Korea's opposition bloc solidified his support scores ahead of the constitutional court's final ruling on the impeachment of President Park Geun-hye, an opinion survey showed on Thursday.

Moon Jae-in, former head of the biggest opposition Minjoo Party, kept a top post in recent opinion poll with 36.1 percent in approval scores, according to a Realmeter poll.

It was slightly down from the previous week, but Moon ranked 1st for 10 straight weeks through this week.

The result is based on a poll of 1,530 voters conducted between Monday and Wednesday. It has 2.5 percentage points in a margin of error.

Moon widened a gap in approval rating with Prime Minister Hwang Kyo-ahn, whose approval scores declined 0.7 percentage points from the previous week to 14.2 percent.

Hwang, who is serving as acting president following Park's impeachment on Dec. 9, emerged recently as the best hope among conservative voters because there is no outstanding rival coming from the conservative camp.

The ruling Liberty Korea Party lost public support as President Park was being implicated in a corruption scandal that led to her impeachment.

The constitutional court announced its plan to make a final ruling Friday about whether to permanently remove Park from office or reinstate her.

If six out of eight justices uphold the impeachment motion, Park will be forced out office. If over three judges reject the motion, she will be reinstated immediately.

Governor Ahn Hee-jeong of South Chungcheong province, who is affiliated with the Minjoo Party, ranked third in approval scores with 12.9 percent, followed by Mayor Lee Jae-myung of Seongnam city near Seoul who gained 10.5 percent in support scores.