"I'm telling you, I'm clean as a whistle, and I think that the testing will prove that," Armstrong said.
The Tour of California is Armstrong's first event on his native soil since the seven-time Tour de France winner began his comeback.
However, the news conference took a dark turn when Armstrong told a reporter from The Sunday Times in London that "You're not worth the chair you are sitting on."
Paul Kimmage, who is well-known for his anti-doping stance and the book "Rough Ride: Behind the Wheel with a Pro Cyclist," said he asked for interview and didn't get one.
"Lance, you've spoken recently about the return of Ivan Basso and Floyd Landis after their suspensions and compare them to David Miller, and that they should be welcomed back in the way that David Miller was welcomed back.
It was one obvious difference in that David Miller has been pronounced in his anti-doping stance, where these guys have admitted to nothing. What is it about these dopers that you seem to admire so much?" Kimmage asked.
Armstrong bristled at the question.
"The reason why you didn't get it, Paul ... when I decided to come back, for what I think is a very noble reason ... you said, folks, the cancer has been in remission for four years, but our cancer has now returned, meaning me," Armstrong said.
Armstrong said he's "here to fight this disease."
"So I think it goes without saying, no, we're not going to sit down and do an interview," Armstrong said. "And I don't think anyone in this room would sit down for this interview. You're not worth the chair you are sitting on with a statement like that, with a disease that touches everyone around the world."
However, Armstrong begrudgingly answered Kimmage's question.
"David, who I admire greatly ... was caught with his hand in the cookie jar. Is it heroic now that he has confessed? Some would say so. I applaud him for being back. ... Floyd, on the other hand, while some would say there's some evidence against him, there's also a lot of evidence in his favor. ... Floyd does not believe that he is guilty. So to appease people like you and others, he can't confess. He doesn't feel like he's guilty. He doesn't feel like he broke the rules.
You can't do that just to get people off your back. ... As a society, are we supposed to forgive and forget and let people get back to their job? Absolutely. I'm not sure I will ever forgive you for that statement. And I'm not sure that anyone around the world that has been affected by this disease will forgive you."
Kimmage argued that he had raced for many years, and a moderator moved on to another reporter's question.
Armstrong said that although he doesn't know as much about baseball as he does football, he is aware of the issues surrounding doping in sports and was tested 16 to 17 times last year.
Asked his opinion on the latest round of steroid revelations in baseball surrounding Alex Rodriguez and Miguel Tejada, Armstrong -- the subject of years of doping suspicion -- professed hope that cycling's more stringent regulation and his personal testing regimen have reduced fans' disbelief in the sport's top athletes.
"I always push for a global standard where everybody plays by the same rules," Armstrong said.
Sacramento Mayor Kevin Johnson, who was also at the news conference, gave Armstrong a "Kevin Johnson For Mayor" bracelet during the event.