South Korean opposition leader Lee Jae-myung, widely seen as the frontrunner in a presidential by-election triggered by the removal of President Yoon Suk Yeol last week, officially announced a presidential bid on Thursday, vowing to heal a starkly divided nation through economic growth.

Lee, who narrowly lost the 2022 election to Yoon, led the liberal Democratic Party’s campaign to oust the former president over his December declaration of martial law.

Lee recently stepped down as the party’s chairman to focus on campaigning for the June 3 election. He is considered the clear frontrunner in party's primary. Kim Dong-yeon, the Democratic governor of Gyeonggi province and a longtime financial policymaker, also told reporters on Wednesday that he intends to run for president.

Yoon’s downfall has left the conservative People Power Party in disarray, with roughly 10 politicians expected to seek the nomination, reflecting a split between Yoon loyalists, who still control the party’s leadership, and reformists calling for a fresh start.

In a video message, Lee said that Yoon’s martial law saga exposed the country’s deep divisions and social conflicts, and argued that the root cause was a widening rich-poor gap. He promised aggressive government spending to jolt economic growth and ease income polarisation.

“We have more than we did in the past, but wealth is too concentrated in certain areas,” Lee said.

“With economic growth rates declining worldwide, it has become difficult to maintain and develop an economy solely on the strength of the private sector. However, with government-led talent development and extensive investments in technological research and development, we can revive the economy.”

He also said it was crucial to maintain a robust alliance with the United States and to pursue three-way cooperation with Japan, but he stressed that South Korea’s national interest should come first in “every decision”.

Lee, who has served as a lawmaker, provincial governor and city mayor, is adored by supporters for his outspoken style and has long positioned himself as an anti-elitist.

His critics view him as a populist who stokes division and demonises conservative opponents, while failing to offer realistic funding plans to achieve his ambitious goals. (AP)