By Ju-min Park

SEOUL (Reuters) - Christian groups in South Korea called on Friday for pop icon Lady Gaga to cancel her concert here saying it was "pornographic" and promoted homosexuality.

South Korea's government has already bowed to public pressure and banned under-18s from attending the 26-year old's concert, but protesters gathered outside the venue said that was not enough.

"Some people can accept this as another culture but its impact is huge beyond art and debases religions. Even adults can't see her performance which is too homosexual and pornographic," said Yoon Jung-hoon, a reverend who organized the "Civilians Network against the Lady Gaga Concert".

The singer's hit song "Born This Way" celebrates the empowerment of gay men and women, something that many in South Korea, which is the second most Christian country in Asia after the Philippines, say is an immoral lifestyle.

Lady Gaga, whose real name is Stefani Joanne Angelina Germanotta, has been in Seoul for close on a week as she kicks off her Asian "Born this Way" tour.

"Nervous for tonight. Hope I make little monsters happy and proud," she Tweeted a few hours before the concert kicked off.

Yoon's group claims to have collected 5,000 supporters on Facebook against the concert and some have also called for a boycott of the sponsor Hyundai Card. The protesters have also threatened a boycott of its parent company, Hyundai Motor Co, South Korea's largest car-maker.

"The Christian Council of Korea with 12 million Christians and 55,000 churches will take all kinds of measures such as boycotting Hyundai Card to root out this kind of obscene culture," the council said in a statement.

Yoon said he would attend the concert to "monitor" the performance for homosexual content that could corrupt young people.

(Reporting by Ju-min Park; Editing by David Chance and Sanjeev Miglani)