SEOUL, June 4, 2025 (AFP) - South Korea's centre-left Lee Jae-myung was declared president Wednesday after winning a snap election, taking the helm of a nation deeply divided after his predecessor's disastrous attempt to declare martial law.

Lee won a thumping victory over conservative Kim Moon-soo, of disgraced ex-president Yoon Suk Yeol's former party, with his term beginning immediately after the National Election Commission certified the vote tally early Wednesday.

Lee secured 49.4 percent of the vote, far ahead of Kim on 41.2 percent -- who was hampered by party infighting and a third party candidate splitting the right-wing vote.

South Korea typically has a longer transition period, but because Lee's victory came in a snap election after Yoon's impeachment, his term began right away.

"The National Election Commission hereby declares Lee Jae-myung of the Democratic Party as the elected president," its chairperson Roh Tae-ak said early Wednesday.

Kim had conceded hours earlier, as final votes were still being counted. Official results showed he had no chance of winning.

Lee assumed office just hours before the United States was set to slap tariffs of 50 percent on South Korea's crucial steel and aluminium exports -- although markets reacted favourably to the election, with the benchmark KOSPI and the won rising Wednesday.

Lee takes charge of a nation still reeling from the turmoil sparked by Yoon's martial law declaration in December and the rise of the far right in its aftermath -- a development experts say has deeply shaken the country's collective sense of democracy.

In a speech to supporters early Wednesday, Lee urged South Koreans to move "forward with hope and make a fresh start from this moment on".

He also promised to "pursue dialogue, communication, and cooperation" with North Korea, with whom the South remains technically at war, "to find a path toward peaceful coexistence and shared prosperity".

Lee started his first day in office speaking to the military's top commander, formally confirming the transfer of operational control of the country's armed forces.

He urged soldiers to "maintain a firm readiness posture" against any North Korean threats, and commended the country's military for their conduct during the martial law crisis, saying they'd "helped prevent the country from descending into further chaos".

Lee then headed to the National Cemetery, where many of the country's former heads of state are buried, to pay his respects.

A modest inauguration ceremony will likely follow at the National Assembly -- the same site where Yoon deployed armed troops on the night he attempted to suspend civilian rule.

- Blue House calling? -
It will be a muted affair with just a few hundred guests expected to attend, in contrast to the large outdoor events held after regularly scheduled elections, which often attract tens of thousands.

The new leader will then head to the presidential office to begin naming key cabinet members.

Ex-president Yoon moved the seat of power from the traditional Blue House to a hastily converted government building in Yongsan, which Lee previously said he would not use.

The focus will be on who is tapped to serve as presidential chief of staff, prime minister and director of the National Intelligence Service.

The day is expected to end with a flurry of congratulatory phone calls from world leaders, with US President Donald Trump likely to be the first on the line.

Trump's top diplomat, Marco Rubio, was swift to offer his own congratulations and voice hope for working with Lee, who previously has sought greater distance from the United States.

Washington's alliance with Seoul was "ironclad", the US secretary of state said in a statement, citing "shared values and deep economic ties".

Lee comes to power with his party already holding a parliamentary majority -- secure for the next three years -- meaning he is likely to be able to get his legislative agenda done.

Lee is also likely to herald a major shift in South Korea's foreign policy, Gi-Wook Shin, a sociology professor at Stanford University, told AFP.

"Lee is expected to prioritize the alliance with the United States while simultaneously seeking engagement with both China and North Korea," he said.

That marks a departure from the policies of predecessors including Yoon, "who largely focused on North Korea and the United States, respectively".