Six months to the day after ex-president Yoon Suk Yeol plunged the country into crisis with his disastrous declaration of martial law, an exit poll by South Korea's three major broadcasters showed Lee of the left-leaning Democratic Party with 51.7 percent of the vote.

Challenger Kim Moon-soo of the conservative People Power Party (PPP) was on track to win 39.3 percent, the poll showed.

Hundreds of Lee's supporters broke into cheers outside the National Assembly, as party officials watching from inside the parliament started a chant of "Lee Jae-myung" as the exit poll results were announced.

For weeks, major polls had put Lee well ahead of Kim -- Yoon's labour minister -- who struggled with party infighting and failed to convince a third party candidate to unify and avoid splitting the right-wing vote.

After months of turmoil and a revolving door of lame-duck acting leaders, many South Koreans said they were eager for the country to move forward.

"I hope the next president will create an atmosphere of peace and unity rather than ideological warfare," cab driver Choi Sung-wook, 68, told AFP as he cast his ballot.

Noh Min-young, 20, who has been protesting in the streets since Yoon declared martial law, told AFP she was "relieved" at the exit poll projection.

"It's been a tough road. I'm happy because it feels like we've seized the opportunity that so many people fought for over the past six months."

South Korea's next leader will take office almost immediately -- as soon as the National Election Commission finishes counting the votes and validates the result, likely early Wednesday.

He will face a bulging in-tray, including global trade vicissitudes chafing the export-driven economy, some of the world's lowest birth rates and an emboldened North Korea rapidly expanding its military arsenal.

- 'Turning point' -

But the fallout from Yoon's martial law declaration, which has left South Korea effectively leaderless for the first months of US President Donald Trump's tumultuous second term, was the decisive factor in the election, experts said.

The vote was "largely viewed as a referendum on the previous administration," Kang Joo-hyun, a political science professor at Sookmyung Women's University, told AFP.

Yoon's impeachment over his martial law bid, which saw armed soldiers deployed to parliament, made him the second straight conservative president to be stripped of office after Park Geun-hye in 2017.

"Lee's victory signals that the Korean public rejects illiberal and undemocratic measures such as martial law," Gi-Wook Shin, a sociology professor at Stanford University, told AFP.

"This moment will likely be remembered as a peculiar and consequential turning point in South Korea's political history."

But Lee's success is due as much to his rivals' failings as his own strengths, said Minseon Ku, a postdoctoral researcher at the William & Mary Global Research Institute.

"Lee has a criminal record and was involved in several political and personal scandals, which made him deeply unpopular among many voters in the 2022 presidential election," Ku said, referring to Lee's unsuccessful run for top office, when he lost to Yoon by a narrow margin.

His rise to the presidency is "a reflection of the deep political turmoil South Korea has been experiencing".

South Korean presidents serve a single five-year term.

- High turnout -

Turnout in the June 3 poll was high, running at around 77.8 percent by late afternoon, officials said -- the highest in nearly two decades.

Ballot counting stations swung into action after polls closed at 8:00 pm (1100 GMT), AFP reporters saw, with boxes of ballots arriving at the Seoul National University Gymnasium in Gwanak-gu district.

On election day, Seoul streets were peaceful as people made the most of good weather and a public holiday, but police issued the highest level of alert and deployed thousands of officers to ensure the election and inauguration Wednesday proceed smoothly.

Lee -- who survived an assassination attempt last year -- has been campaigning in a bullet-proof vest and delivering speeches behind a glass protective shield.

With Lee's projected victory, the country's left wing Democratic Party has "strengthened their hold on moderately conservative urban men and women in their 30-40s", Vladimir Tikhonov, Korean Studies professor at the University of Oslo, told AFP.

These voters "just want some normalcy and gradual improvement of the welfare system", he said.

"Yoon's coup attempt scared these people, and for the time being they are not going to return to the PPP."