A former senior North Korean diplomat won a parliamentary seat in Seoul Thursday, becoming the first defector to be directly elected by South Korean voters.

Thae Yong Ho, who was Pyongyang's deputy ambassador to London before fleeing to the democratic South in 2016, shed a tear as he sang South Korea's national anthem after his win was confirmed in the early hours.

He stood for the main conservative opposition United Future Party (UFP) and took 58.4 percent of the vote in his Gangnam constituency, the wealthy Seoul district made famous by rapper Psy.

"Today is a very historic day not only for me but also for the history of Korea," Thae told AFP shortly after his victory was announced.

Officially he ran as Tae Ku-Min -– a pseudonym he registered under on arrival in the South to initially make it harder for Pyongyang's agents to track him down.

Thae comes from a family with good revolutionary credentials, but fled his post in London in August 2016 after becoming disillusioned with Kim Jong Un's regime.

He has since become a prominent and outspoken critic of Pyongyang and the engagement approach pursued by South Korean President Moon Jae-in.

Moon's Democratic party secured the largest absolute majority in the National Assembly since the advent of democracy in 1987, on a turnout of 66.2 percent -- the highest at a parliamentary election for 28 years.

Thae said his victory will give hope to the North Korean people and the elite, and demonstrate the "real nature of free democracy of South Korea".

His election could be a window to enable North Koreans to "understand how free and democratic election would go", he said.

North Korea holds an election every five years for its rubber-stamp legislature, known as the Supreme People's Assembly, but the ballot papers for the vote each only have one name.

"One day in the future I'm sure North Korean people will adopt the same method of election process in North Korea," Thae told AFP in an interview during his campaign.

Ji Seong-ho, the crutch-wielding defector hailed by US President Donald Trump in his 2018 State of the Union address, won a proportional representation seat for the UFP in Wednesday's vote.

About 33,000 North Koreans have fled to the South in the past two decades, but it is rare for high-level officials to defect.

The North's state media has denounced Thae as "human scum" and accused him of embezzling state funds, raping a minor and spying for South Korea in exchange for money.