Several information technology experts predicted there would be even more technical problems as online systems confront premium payments, changes in eligibility and other complex tasks.

“Is this just the tip of the iceberg?” asked Harold Tuck, former chief information officer for San Diego County. “There ought to have been better beta testing of the systems, and these errors wouldn’t have come up.”

Bill Curtis, chief scientific officer at CAST, a New York-based firm that analyzes information technology systems, said the heavy traffic probably explained many of the problems. “When you have this kind of volume, it exposes all kinds of weaknesses,” he said.

Nevada’s marketplace, Nevada Health Link, had 60,000 unique visitors on its first day, spokesman C.J. Bawden said.

Many visitors may be curiosity seekers, an analysis advanced by critics of the law suggested.

“I think if you subtract out members of Congress and their staff and reporters who called in those first 48 hours, the numbers will be considerably lower,” Rep. Michael C. Burgess (R-Texas) told CNN last week.