The confession was a stunning reversal for Armstrong after years of public statements, interviews and court battles in which he denied doping and zealously protected his reputation.
Even before the taping session with Winfrey began, Armstrong's apology suggested he would carry through on promises over the weekend to answer her questions "directly, honestly and candidly."
The cyclist was stripped of his Tour de France titles, lost most of his endorsements and was forced to leave the foundation last year after the U.S. Anti-Doping Agency issued a damning, 1,000-page report that accused him of masterminding a long-running doping scheme.
About 100 staff members of the charity Armstrong founded in 1997 gathered in a conference room as Armstrong arrived with a simple message: "I'm sorry." He choked up during a 20-minute talk, expressing regret for the long-running controversy over performance-enhancers had caused, but stopped short of admitting he used them.
Before he was done, several members were in tears when he urged them to continue the charity's mission of helping cancer patients and their families.
"Heartfelt and sincere," is how Livestrong spokesman Katherine McLane described his speech.
Armstrong later huddled with almost a dozen people before stepping into a room set up at a downtown Austin hotel.
The group included close friends and advisers, two of his lawyers and Bill Stapleton, his agent, manager and business partner. They exchanged handshakes and smiles, but declined comment when approached by a reporter.
No further details about the interview were available immediately because of confidentiality agreements signed by both camps. But Winfrey promoted it as a "no-holds barred" session, and after the voluminous USADA report which included testimony from 11 former teammates she had plenty of material for questions. USADA chief executive Travis Tygart, a longtime critic of Armstrong's, called the drug regimen practiced while Armstrong led the U.S. Postal Service team, "The most sophisticated, professionalized and successful doping program that sport has ever seen."
Armstrong also went after his critics ruthlessly during his reign as cycling champion, scolding some in public and waging legal battles against others in court.
Betsy Andreu, the wife of former Armstrong teammate Frankie Andreu, was one of the first to publicly accuse Armstrong of using performance-enhancing drugs. She called news of Armstrong's confession "very emotional and very sad," and got choked up when asked to comment.
"He used to be one of my husband's best friends and because he wouldn't go along with the doping, he got kicked to the side. Lance could have a positive impact if he tells the truth on everything," she said.
Storylines Armstrong opens up on doping
R Oprah Winfrey's interview Monday with Lance Armstrong will be broadcast on her TV network Thursday, 7 p.m.
• Sources say Armstrong admits to doping.