In widely expected results cementing his popularity, former human rights lawyer Moon Jae In was formally nominated yesterday as the main opposition Democratic Party's presidential candidate for the May 9 elections.

Mr Moon, 64, garnered 57 per cent of votes cast in the party's four primaries held in the past week, beating his rivals South Chungcheong Governor An Hee Jung, Seongnam Mayor Lee Jae Myung and Goyang Mayor Choi Sung.

In his acceptance speech after the final primary, held in a baseball stadium in Seoul, Mr Moon pledged to end division in South Korea.

He also vowed to reinvigorate the sluggish economy, enhance national security and eradicate corruption, saying: "We must move towards a fair society."

Analysts say Mr Moon's pro-North Korea stance and untested national security policy will prove to be a challenge when official campaigning begins this Sunday.

He has called for inter-Korea talks and objected to the government's decision to deploy a US anti-missile system in the country, even though his positions run counter to the US-South Korea alliance crucial to keeping Pyongyang in check.

Although a multi-cornered fight is expected, pundits say it will most likely come down to liberal candidate Mr Moon and former software tycoon Ahn Cheol Soo, whose candidacy for the centrist opposition People's Party is due to be revealed today.

Mr Moon is making his second bid for the presidency, after losing 48-52 to Park Geun Hye in the 2012 elections.

The former aide to the late liberal president Roh Moo Hyun has held the top spot in opinion polls since late last year, scoring an approval rating of 34.9 per cent in the latest Realmeter poll released yesterday.

Mr Ahn's approval rating has risen sharply in the past few weeks - to 19 per cent in the latest Gallop Korea poll, almost double the figure a week ago, as voters turned to him to foil a likely Moon presidency.

Speculation has also grown that Mr Ahn might form a coalition with the conservative ruling Liberty Korea Party and the splinter Bareun Party, so as to run a more effective campaign against Mr Moon.

Mr Moon, while hugely popular with younger liberals, risks alienating older conservatives with his pro-North policies.

"Moon promises a radical break from the present administration, but a lot of it could be political posturing," said Dr Go Myong Hyun of The Asan Institute of Policy Studies think-tank. "Cancelling missile defence will upset the United States a great deal but won't make China an immediate friend."