"Gangnam style" sex crime: K-pop scandals uncover dark side of Seoul’s flashiest district

* Scandals ensnare K-pop stars, top clubs, and police officials

* Clubs alleged to be site of prostitution, sex assault, drugs, and financial crimes

* At least six police investigated for colluding, accepting bribes

By Joyce Lee and Joori Roh

SEOUL, April 14 (Reuters) - On a recent weekend night, the dance floor at one of the hottest clubs in Seoul's swanky Gangnam district held only a few dozen people surrounded by mostly empty tables.

A few months ago, the nightclub would have been packed with hundreds of gyrating men and women, and full tables, many costing 650,000 won ($570) or more for a night of drinking and dancing.

The world was introduced to Gangnam by the 2012 K-pop hit “Gangnam Style,” a parody of the South Korean highlife with a viral tune and amusing dancing that became the first video to reach a billion views on YouTube.

But a wave of sex crimes and other illegal activity has revealed a dark underbelly in the district, driving club-goers and celebrities away.

According to police investigators, a network of pop stars, businessmen and cops are alleged to have colluded and enabled tax evasion, bribery, and prostitution at some of Gangnam's glitziest clubs.

Most seriously, some are being investigated over the use of date rape drugs to incapacitate women and assault them, sometimes filmed by hidden cameras.

"There aren't many people coming to Gangnam (clubs) right now," a worker told Reuters at a club that was relatively quiet, despite not being implicated in any of the allegations. "There's an investigation on."

The scandals have already led to the resignation of four K-pop stars, the closure of one of Gangnam’s most lucrative club, and investigations into at least six police officers suspected of colluding with club operators.

President Moon Jae-in called for a thorough investigation, saying the Gangnam club cases suggest possible collusion between police, tax authorities and a new privileged class including celebrities to engage in illegal operations.

More than 500 people have been investigated for drug use and sexual assault and more than 200 arrested in a nationwide roundup since Feb. 25.

Tax authorities have launched investigations into 21 clubs and host bars for possible evasion.

"If we don't set this right, we cannot call this a just society," Moon said.

SEX CRIME CONCERNS

The investigations, revolving around two Gangnam clubs, Burning Sun and Arena, began late last year when 29-year-old film art director Kim Sang-kyo says he tried to stop an incident of sexual harassment and was attacked by Burning Sun staff, and then abused by police who instead arrested him.