Presidential front-runner Moon Jae-in officially declared his bid for South Korea's top elected office Friday, vowing to restore justice and national unity, ensure the rule of law and strengthen independent defense capabilities.

The former leader of the largest Democratic Party made the announcement in a video clip released on his website, Facebook and YouTube. 

 "(The Republic of Korea) must be a country in which common sense prevails and what should be fair should be regarded as fair... Its society must be one in which justice is seen, heard and felt," Moon said.

 The liberal politician and former human rights lawyer delivered emphatic messages to working mothers, job seekers, military draftees, the disabled, overseas residents and other marginalized citizens.

"(In our society), those working laboriously must not hover around the poverty line, those who stick to the law and principles must be respected and 'different' views must not be dismissed as wrong," he added.

In various recent opinion polls, Moon has maintained an unrivaled lead for the May 9 election. The rare presidential by-election was triggered after the Constitutional Court ruled earlier this month to dismiss President Park Geun-hye over a corruption scandal.

In the wake of the conservative leader's ouster, the country's political landscape has been seen heavily weighted in favor of the liberal bloc.

On the security front, Moon pledged to build a "strong nation that does not fear any other country in the world" based on robust independent defense capabilities. He also sent a message to conscripts, saying that "schools or jobs must await them" after they complete their mandatory military service.

Amid deepening social divisions over the ouster of Park, Moon called for national harmony and integration.

Moon's campaign made three video clips -- two featuring citizens and overseas voters reading Moon's message to the voters and the other in which Moon himself declares his presidential bid.

They have been posted on Moon's campaign website (moonjaein.com) and his social media accounts. (Yonhap)