A combination of high demand and technical glitches seemed to overwhelm the online system early in the day. Federal and state officials were working to address the problems, which led to long waits on government websites and a federal call centre.
Health and Human Services spokeswoman Joanne Peters said that more than 1 million people had visited HealthCare.gov in the last day — five times more users than have ever been on the Medicare.gov at one time.
Kimberly Shockley — logging in from Houston, Texas — and Mike Weaver, who lives in rural southern Illinois, ran into similar glitches: They could not get past the security questions while trying to set up their personal accounts through healthcare.gov.
"I'm frustrated, very frustrated," said Shockley, a self-employed CPA. She spent more than an hour trying to get the security questions to work Tuesday morning without success. When she clicked on a drop-down menu of suggested security questions, none appeared. She then tried to create her own questions, but that didn't work either.
Weaver, a self-employed photographer, said he also ran into problems with the drop-down menus. And when they started working, he still wasn't able to set up his account.
"The first day of something that you know is going to have a lot of bugs, it's not that frustrating," he said. "If it was the last day to sign up ... then I'd be terribly frustrated."
Shockley has health insurance, but is looking for a better plan. Weaver is uninsured.
State-operated sites also experienced glitches. Rhode Island's site opened as scheduled, but was quickly overwhelmed by visitors and went down. A spokesman for the New York Department of Health blamed problems with the 2 million visits to the website in the first 90 minutes after its launch. Washington state's marketplace used Twitter to thank users for their patience.
Exchange officials in Colorado said their website would not be fully functional for the first month, although consumers will be able to get help applying for government subsidies during that time.
Connecticut seemed to be a bright spot. Access Health CT sent out a tweet shortly before noon Tuesday, confirming the marketplace logged 10,000 visitors in the first three hours of operation and 22 enrollments. A family of three was the first to sign up for coverage.
In Portsmouth, N.H., Deborah Lielasus tried to sign up for coverage but got only as far as creating an account before the website stopped working. She said she expected glitches.
Lielasus, a 54-year-old self-employed grant writer, currently spends about $8,500 a year in premiums and more than $10,000 for out-of-pocket expenses because she has a health condition and her only option was a state high-risk insurance pool. She said she expects those costs to decrease significantly.
As excited as she was to sign up, she said, her anticipation was tempered by dismay over the government shutdown that was led by congressional Republicans who want to block the health insurance reforms.
"I'm really happy that this is happening, that this is being launched ... I feel like it's a child caught in the middle of a really bad divorce," Lielasus said.
The nationwide rollout comes after months of buildup in which the marketplaces have been both praised and vilified.
The shutdown will have no immediate effect on the insurance marketplaces that are the backbone of the law, because they operate with money that isn't subject to the annual budget wrangling in Washington.
The marketplaces represent a turning point in the nation's approach to health care, the biggest expansion in coverage in nearly 50 years.
The Obama administration hopes to sign up 7 million people during the first year and aims to eventually sign up at least half of the nearly 50 million uninsured Americans through an expansion of Medicaid or government-subsidized plans.
But if people become frustrated with the glitches in the computer-based enrolment process and turn away from the program, the prospects for Obama's signature domestic policy achievement could dim.
"You've got to launch this thing right the first time," said Robert Laszewski, a consultant who worked 20 years in the insurance industry. "If you don't, financially you will never recover."
Neera Tanden, president of the Center for American Progress, which helped work for passage of the law, cautioned against rushing to judge the marketplace's success on its first-day performance. Numerous observers had predicted bugs and setbacks. Trained outreach workers in many states are having trouble getting the certification they need to start helping people to enrol.
Many states predicted that an initial surge of interest would test the online system, but they expect most people to sign up closer to Dec. 15, which is the deadline for coverage to start Jan. 1. Customers have until the end of March to sign up in order to avoid tax penalties.
Looming as one of the biggest challenges to the law's success is persuading young, healthy people to buy insurance to balance the costs of covering older, sicker Americans.
Under the law, health insurance companies can no longer deny coverage to someone with a pre-existing medical condition and cannot impose lifetime caps on coverage. They also must cover a list of essential services, ranging from mental health treatment to maternity care.
Another obstacle: Nearly three-fourths of people under 65 who lack insurance are unaware that the marketplaces open Tuesday, according to a Kaiser Family Foundation survey released over the weekend.
Spending money to raise that awareness with ad campaigns has varied vastly, with some Republican-led states doing little or nothing to promote the insurance exchanges. Missouri Lt. Gov. Peter Kinder, a Republican, even recently urged residents not to sign up for coverage.
In Florida, Republican Gov. Rick Scott and key lawmakers have pushed back against implementing parts of the law. The Florida Department of Health recently ordered county health departments to prohibit so-called navigators from signing people up for health insurance at those facilities.
But other states are doing more, such as Kentucky, the only Southern state running its own marketplace. Kentucky Gov. Steve Beshear, a Democrat, was an early supporter of the health law.
The state kicked off an $11 million advertising campaign in June, with ads on TV, radio, Internet and newspapers. It will expand Tuesday and continue through the first three months of next year.
"Frankly, we can't implement the Affordable Care Act fast enough," Beshear said.
Associated Press writers Kelli Kennedy in Miami; David Lieb in Jefferson City, Mo.; Kristen Wyatt in Denver; Kathy Matheson in Philadelphia; Erika Niedowski in Providence; Holly Ramer in Portsmouth, N.H.; Ricardo Alonso-Zaldivar in Washington; Carolyn Thompson in Buffalo, N.Y; Susan Haigh in Hartford, Conn.; and Roger Alford in Frankfort, Ky., contributed to this report.
AP Medical Writer Carla K. Johnson can be reached at http://www.twitter.com/CarlaKJohnson .