The disgraced seven-time Tour de France winner received a standing ovation at the Austin, Texas, event and spoke for the first time since the doping scandal engulfed him, telling the star-studded gathering:
"I am humbled by your support. It's been a difficult couple of weeks for me and my family, my friends and this foundation. I've been better but I've also been worse. The mission absolutely must go on. We will not be deterred. We will go forward. Twenty-eight million people around the world need us."
Armstrong stepped down Oct. 17 as chairman of the Livestrong Foundation after the U.S. Anti-Doping Agency released evidence that the American was a drug cheat.
Armstrong has been banned from cycling and stripped of his titles by the USADA. He was also dropped by sponsors Nike and bike maker Trek.
According to the Fairfax media, cycling's governing body, the UCI, was expected to ratify the USADA report on Monday.
Armstrong did not use the public entrance to the event, which according to KXAN Austin was attended by actors Ben Stiller, Sean Penn, Robin Williams and Matthew McConaughey and singer Norah Jones.
Australia's ABC quoted Penn as saying he came to support both Armstrong and the foundation.
"Lance has developed an organization that has become an inspiration to me and to others—and I think it will remain. I think to anyone who looks at this with a clear eye will see it as hypocritical to think otherwise."
According to Radio New Zealand, Armstrong indirectly addressed the doping controversy, saying that the past two weeks "have been difficult."
Mainstream media was banned from the event, however one member of the audience filmed part of his speech and the video had since been uploaded online. Livestrong also said it would upload the speech on YouTube.
The foundation said that donations for the charity had continued flow in, despite the scandal.
"In addition to funds raised this evening, between October 17-19, Livestrong received nearly $240,000 in online donations with an average donation of $94," the foundation said in a statement reported by Agence France-Presse.