But the administration says such talk is overblown. Consumers can move from expensive health plans to more efficient, lower-cost policies, officials say, and critics who focus on premiums do not take account of other provisions of the law that limit how much consumers will spend out of pocket for health care.

The administration predicts that those who get coverage after being uninsured are generally going to be younger and healthier than those who have insurance. That will hold down total costs of coverage for insurers and the government, officials say, and tend to hold down premiums in turn. If premiums do increase, they argue, lower-income people can get federal help.

Either way, both sides face a colossal challenge in planning for one of the most complicated and far-reaching social undertakings ever. On Oct. 1, enrollment starts nationwide for the emerging marketplaces where many millions of Americans can shop for coverage. Mr. Obama’s signature Affordable Care Act of 2010 requires most Americans to have health insurance, and provides subsidies for the needy, to expand coverage to an estimated 27 million uninsured Americans — including seven million through the exchanges in the first year.

Mr. Obama had not been scheduled to attend the meeting, but he presided over the entire hourlong session, which included his chief of staff, Denis R. McDonough, and his health and human services secretary, Kathleen Sebelius. One official called it “productive,” adding that “the president discussed our shared goal of educating Americans about the value of health insurance and the new health insurance marketplaces.”

Among attendees from the industry, which stands to get millions more customers, were Karen M. Ignagni, the head of a trade association for the insurance industry; Chet Burrell of CareFirst BlueCross BlueShield; David M. Cordani of the Cigna Corporation; Scott P. Serota of the BlueCross BlueShield Association, a federation of local Blue Cross and Blue Shield companies; and Joseph R. Swedish of WellPoint.