It's a Psy-cho 'Christmas in Washington

WASHINGTON -- Even the hottest celebrities aren't too cool to sing a Christmas carol or two.

Legendary performer Diana Ross, recording star Demi Lovato and of-the-moment Gangnam Style rapper Psy were among the artists that brought a hearty dose of fa-la-la-la-la to the 31st annual Christmas in Washington on Sunday evening.

Jingle bells were ringing in a holiday-festooned National Building Museum as Christmas classics new and old were sung to an audience that included President Obama and Mrs. Obama. Hosted for the second year in a row by Conan O'Brien, the holiday tradition benefits the Children's National Medical Center.

"It's good to see that both the president and I were given a second term," joked O'Brien . "I am worried that the last person the president wants to see after the last six months is another white guy from Massachusetts."

Other performers included Smash star Megan Hilty, The Voice favorite Chris Mann and American Idol winner Scotty McCreery.

Ross, in a royal blue sequined gown, kicked off the show with a medley that included Sleigh Ride, Jingle Bells and It's the Most Wonderful Time of the Year.

Hilty sang Have Yourself a Merry Little Christmas, Lovato sang the saucy favorite All I Want for Christmas is You and Mann belted out I'll be Home for Christmas. McCreery (who O'Brien noted is "a man blessed with the adult male voice I've always wanted") sang Santa Claus is Back in Town.

Sporting a red-sequined top and red pants, Internet sensation Psy began his portion of the performance singing the classic The Christmas Song — but that didn't last long. After stopping to question why he was there, he launched into his bass-thumping Gangnam Style, which turned into Christmas Style. The crowd, full of Washington VIPs including CNN's Wolf Blitzer, ate it up with looks of both amusement and bewilderment.

It's a far cry from Friday, when the South Korean rapper, born Park Jae-sang, apologized for using what he says was "inflammatory and inappropriate language" during anti-U.S. protests at concerts in 2002 and 2004.

All of the performers came together for a closing medley, which featured Ross capping things off with Amazing Grace.

The evening ended with the first family joining all of the performers in singing Hark, the Herald Angels Sing on stage.

"Tonight is a chance to get in the Christmas spirit ... it's also a chance to make a difference in the lives of some very brave young people," Obama said.

The star-studded concert will premiere at 8 p.m. ET/PT on Dec. 21 on TNT. This will mark TNT's 14th year presenting Christmas in Washington.