Jacquelyn Martin / AP

From left, Malia Obama, President Barack Obama and performers Jennifer Hudson, Victoria Justice and Justin Bieber, stand on stage before singing "Hark! The Herald Angels Sing" at the annual Christmas in Washington taping at the National Building Museum in Washington D.C. on Dec. 11, 2011.

After singing for and with the First Family, Justin Bieber tweeted:

Great night! President Obama is one cool Prez. He told me he was "chillin." #PresidentialSWAG

Reuters reports from WASHINGTON:

Jacquelyn Martin / AP

President Barack Obama struck a pious tone on Sunday at a Christmas concert featuring teen pop star Justin Bieber and other singers that the president attended with his wife and daughters.

Comedian Conan O'Brien, hosting the TNT "Christmas in Washington" show, joked that at this time of year, thoughts turn to a miracle child worshiped by millions worldwide.

"I'm speaking of course, of Justin Bieber," O'Brien told the audience at Washington's National Building Museum, introducing the Canadian-born singer.

Obama, who attended a church service with his family on Sunday morning - which he does only rarely - joked and played with young patients from a Washington-area hospital before the show but was serious in his remarks to the crowd. Continue reading.

Related content:

• Slideshow - Christmas at the White House
• More pictures of Barack Obama and Justin Bieber on PhotoBlog.