Lady Gaga and Tony Bennett at final Obama ball

Pop diva Lady Gaga and veteran crooner Tony Bennett spanned the generations as President Barack Obama offered a final thank you to his army of campaign volunteers at his last inaugural ball. Obama and First Lady Michelle Obama appeared at a staff ball, which drew more than 10,000 guests, a day after official inaugural ceremonies marking the start of his second White House term. \"My main job here tonight is really simple -- it\'s just to say thank you,\" Obama said, in a valedictory appearance before volunteers from his nationwide grassroots campaign battalions which helped him win two presidential terms. \"All of you have come to represent for me and Michelle our deepest hopes for America,\" said Obama, a former community organizer, who built the most formidable campaign machine in the history of US politics. \"It makes me know that America\'s future is in good hands. As long as all of you understand the immense and incredible power that you possess when you work together, when you join voices.\" Senior aides to Obama announced last week that they would build a new organization pulled from his campaign infrastructure which would be used to back his second-term agenda. The president ended his appearance with a rendition of his campaign mantra \"Fired Up, Ready to Go\" which dated to his 2008 White House race, prompting the crowd to shout the slogan back at him. \"I think Lady Gaga\'s fired up too,\" Obama said. The event was closed to press photographers, but pictures tweeted from inside showed Lady Gaga and Bennett sharing the stage singing an unidentified song. Proceeds from the ball at the Washington National Convention center went to a memorial fund for young campaign worker Alex Okrent who died suddenly during Obama\'s campaign.