SEOUL (REUTERS) - A former North Korean diplomat has won a parliamentary seat in South Korea's swankiest district, four years after fleeing a London embassy and defecting to the South.

Mr Thae Yong Ho was Pyongyang's deputy ambassador to the United Kingdom, where he managed secret funds for leader Kim Jong Un until he fled to the democratic, wealthy South in 2016.

Mr Thae, 55, won one of three constituencies in Seoul's glittering Gangnam district for the main opposition conservative party in Wednesday's (April 15) parliamentary election, beating a former four-term ruling party lawmaker by nearly a 20 per cent margin.

"I was dearly afraid if the Gangnam residents would take a guy from the North," a smiling Mr Thae said in an acceptance speech shared on his YouTube channel, as crowds of voters gathered in his campaign office, chanting his name and cheering.

"But so many people here gave me strength - they held my hand, welcomed me to Gangnam and promised to vote for me," he said in a hoarse voice after weeks-long rallies.

Gangnam, a wealthy, conservative district known for its boutiques, high-end bars and luxury homes, shot to international renown through musician Psy's 2012 K-pop hit Gangnam Style.

Mr Thae, who had worked in South Korea as a foreign policy expert before launching a bid for Parliament in February, is the first North Korean refugee to become a lawmaker representing a constituency in South Korea.

Mr Cho Myung-chul, a former North Korean professor, served in Parliament from 2012-16 on a party endorsement but without any constituency.

The election drew unprecedented levels of participation from North Korean defectors challenging what they saw as the flawed cross-border policy of President Moon Jae-in.

Defector groups have complained that Mr Moon's administration cut funding, ignored human rights and suppressed anti-Pyongyang activism in favour of pursuing reconciliation with the Kim regime.

Mr Ji Seong-ho, a defector invited to Washington in 2018 to attend United States President Donald Trump's State of the Union address, was also elected to the Parliament as a proportionate representative.

'NO ROOTS'

But Mr Thae's political foray was far from easy, facing a backlash from some citizens and even his party's campaign chief, Mr Kim Chong-in, who said Mr Thae was not suitable to represent the district because he had "no roots in the South".

Mr Kim later apologised and endorsed Mr Thae.

"His landslide victory means the people of Gangnam didn't really care that he's a defector. They seem to believe that he's more loyal to the South, not the North," said Professor Lim Seong-ho from Kyung Hee University in Seoul.

Critics also raised questions about Mr Thae's wealth when he listed more than 1.8 billion won (S$2.09 million) in assets.

Mr Thae has said he made the money giving lectures and writing books and papers, and paid more than 100 million won in taxes every year.

His two memoirs were big hits. The first of them, in which he described how Pyongyang covertly amassed foreign currency via diplomatic missions as well as the duplicity of its denuclearisation commitment, sold more than 200,000 copies in South Korea and was published in several languages.

North Korea's state media has criticised Mr Thae's defection, calling him a "human scum" and criminal who had embezzled state funds and sexually assaulted a minor.

Mr Thae has flatly denied those claims and vowed to continue his political activities even after police stepped up security due to safety concerns.

"I had risked my life to come to the Republic of Korea seeking free democracy and values in market economy," Mr Thae said.

"I will solve regional issues with the market principles... while pushing for North Korea policy that better reflects the reality there and matches the pride of the South Korean citizens."