South Korea's new liberal President Lee Jae-myung pledged on Wednesday to raise the country from the near destruction caused by a martial law attempt and revive an economy besieged by global protectionism that is threatening its very existence.

Lee's decisive victory in Tuesday's snap election stands to usher in a sea change in Asia's fourth-largest economy, after backlash against a botched attempt at military rule brought down Yoon Suk-yeol just three years into his troubled presidency.

Lee faces what could be the most daunting set of challenges for a South Korean leader in nearly three decades, ranging from healing a country deeply scarred by the martial law attempt to tackling unpredictable protectionist moves by the United States, a major trading partner and a security ally.

"A Lee Jae-myung government will be a pragmatic pro-market government," he said after taking the oath of office at parliament, a location where six months ago he jumped over the perimeter wall to enter the chamber and avoid martial law troops barricading it to vote down the decree.

He promised deregulation to spur innovation and growth in business and pledged to reopen dialogue with North Korea while maintaining a strong security alliance with the United States and bringing balance to diplomacy.

"It is better to win without fighting than to win in a fight, and peace with no need to fight is the best security," he said on the country's often violent ties with rival North Korea.

Lee was officially confirmed earlier as president by the National Election Commission and immediately assumed the powers of the presidency and commander in chief, speaking with the top military leader to receive a report on defence posture.

With all the ballots counted, Lee won 49.42 percent of the nearly 35 million votes cast while conservative rival Kim Moon-soo took 41.15 percent in the polls that brought the highest turnout for a presidential election since 1997, official data showed.

Lee has said he would address urgent economic challenges facing the country on the first day in office with a focus on the cost-of-living concerns affecting middle and low-income families and the struggles of small business owners.

He also faces a deadline set by the White House on negotiating import duties that Washington has blamed for a large trade imbalance between the countries.

South Korean stocks rallied on Wednesday morning, with the benchmark Kospi rising more than 2 percent to its highest in 10 months, with the financial sector leading the gain on expectations of market reform by Lee. Renewable energy stocks also rose. Lee has pledged a shift to a greener energy mix. (Reuters)