After more than a decade of outrunning accusations that he had doped during his celebrated cycling career, Lance Armstrong, one of the best known and most accomplished athletes in recent history, surrendered on Thursday, ending his fight against charges that he used performance-enhancing drugs.

Armstrong, who won the Tour de France an unprecedented seven straight times, said that he would not continue to contest the charges levied against him by the United States Anti-Doping Agency, which claimed that he doped and was one of the ringleaders of systematic doping on his Tour-winning teams.

He continued to deny ever doping, calling the antidoping agency’s case against him “an unconstitutional witch hunt” and saying the process it followed to deal with his matter was “one-sided and unfair.”

“There comes a point in every man’s life when he has to say, ‘Enough is enough,’ ” Armstrong said in a statement. “For me, that time is now.”