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  (Photo: Reuters/Kim Hong-Ji)
  Singer Psy (C) performs at the Seoul Plaza in front of Seoul city hall October 4, 2012. Local media reported that about 80,000 fans gathered to cheer Psy at a free concert staged to thank fans in Seoul. The quirky dance track "Gangnam Style" became a global sensation that has clocked up more than 300 million views on YouTube since its release in July.

Psy, the rapper well known for YouTube sensation "Gangnam Style" and its billion views on the site, performed for President Barack Obama at a holiday concert Sunday. Before the concert, however, the Korean entertainer made sure to apologize for his previous "inflammatory and inappropriate" anti-American lyrics.

Psy apologized Friday for a 2004 incident in which he violently recommended to kill American soldiers and their families "slowly and painfully" because of their presence in Iraq. The apology helped smooth things over for the "Christmas in Washington" concert, which benefits the Children's National Medical Center.

"As a proud South Korean who was educated in the United States and lived there for a very significant part of my life, I understand the sacrifices American servicemen and women have made to protect freedom and democracy in my country and around the world," the rapper, born Park Jae-sang, stated. "The song I was featured in- from eight years ago- was part of a deeply emotional reaction to the war in Iraq and the killing of two innocent Korean civilians that was part of the overall antiwar sentiment shared by others around the world at that time."

"While I'm grateful for the freedom to express one's self I've learned there are limits to what language is appropriate and I'm deeply sorry for how these lyrics could be interpreted. I will forever be sorry for any pain I have caused anyone by those words," he continued. "While it's important we express our opinions, I deeply regret the inflammatory and inappropriate language I used to do so."

With the apology apparently accepted, Psy joined other entertainers like Diana Ross, Demi Lovato, "American Idol" winner Scotty McCreery and comedian Conan O'Brien for the concert. The Korean rapper wore a red sparkling outfit during "Gangnam Style."

President Obama and his family presented books to former patients of the Children's National Medical Center. Obama also had some remarks about the Christmas spirit.

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"Tonight is about Conan and Diana Ross and Santa and all the other talented folks on this stage. But it's also about the Americans who are spending this holiday in a hospital bed, or a shelter, or protecting our freedom on a battlefield far from home," he said.

"Let's keep them in our prayers, and follow Christ's calling to love one another as He has loved all of us," Obama added.