Obama said the opportunity to access affordable insurance is life-changing for those who could not do so before the launch of the exchanges, now open for enrollment for six months starting Tuesday. As a sign of how eager Americans were to get started, Obama said more than 1 million people had visited the website before 7 a.m. EDT — exceeding expectations and, in some cases, slowing down the computer systems.

"This is life-or-death stuff," Obama said in the White House Rose Garden, flanked by Americans who plan to enroll through the exchanges. He said tens of thousands of Americans die each year for lack of health insurance, and others go bankrupt. "Today we begin to free millions of our fellow Americans from that fear."

Obama urged Americans to call in or go online, promoting an online system that he said will offer more choices, more competition and lower prices. For that to work, the Obama administration needs tens of millions of Americans — mostly younger, healthy people — to sign up of offset the costs of patients whose health care costs more.