While no major glitches were immediately reported, the site was slow to load once users clicked on a tab that said "Start Here." Officials had predicted delays on the first day and said many would merely be seeking information rather than signing up for coverage, which begins in the new year.

"Like anything when you first start, you've got to adjust a little bit," said Pat Macht, a spokeswoman for Covered California . "The system's not been down, but it might have had some slower response time. But people are signing up."

Gov. Jerry Brown, meanwhile, announced he had signed a package of bills to help implement the new law and expand the state's Medi-Cal program for those who are too poor to pay for the subsidized insurance.

"While extreme radicals in Washington shut down our government, here in California we're taking action to extend decent health care to millions of families," Brown said in a statement, referring to the impasse in Congress that has led to a partial shutdown of federal government operations.

California officials said the shutdown had no effect on the state exchange.

Some 5.3 million Californians are eligible to apply for coverage under President Barack Obama's health law, known as "Obamacare," about 1.4 million of them through expanded access to the state's health insurance program for the poor, Medi-Cal.

The first completed health insurance application was taken at 8:04 a.m., said Carene Carolan, deputy director of the Rancho Cordova service center.

"We are getting a huge, huge volume of calls," she said.

Officials said figures on how many people enrolled would not be available until Nov. 15, as agents need time to process and approve the applications.

Lee, the executive director, was joined at the Rancho Cordova call center Tuesday by state Health and Human Services Agency Secretary Diana Dooley, lawmakers who support the federal Affordable Care Act and the president of the state's largest public employee union, which represents the call center workers.

"This is absolutely the most important piece of health care legislation that this nation has ever seen," said state Sen. Ed Hernandez, D-Covina, an optometrist who attended the opening. "I am ecstatic. Last night, I didn't sleep very well just thinking about this."

The Affordable Care Act will dramatically change the way many Americans get health insurance and marks the most extensive change to the nation's health care system since Medicare and Medicaid became law in 1965.

Under the law, consumers who have previously been rejected for private insurance because of a pre-existing condition will be able to enroll, and those who cannot afford to buy their own insurance can receive government-subsidized premiums. Annual out-of-pocket expenses will be capped, and insurance companies cannot impose a maximum lifetime benefit.

California is being seen as a laboratory for the nation's health care overhaul, and call center workers have been training for months to answer questions. The three call centers — in the eastern San Francisco Bay Area city of Concord, Fresno and Rancho Cordova — were flooded with calls even before Tuesday's opening.

Latinos, who make up nearly half of the 5.3 million uninsured Californians who are eligible to apply, have been expressing particular interest, said Santiago Lucero, a spokesman for Covered California. Information is available in 13 languages, but those who are in the country illegally are not eligible for coverage.

Actual coverage through insurance purchased on the exchanges will start Jan. 1. Beginning in 2014, virtually all Americans will be required to have health insurance or pay an annual penalty to the government. The penalty for an individual starts at $95 in 2014 but rises to a minimum of $695 by 2016.

Californians who have health coverage through an employer will be largely unaffected by Tuesday's opening of the exchange, which is primarily for the poor, low-income earners, and individuals and families who already buy their own health insurance.

A part of the exchange designed for small business owners to buy insurance did not open online Tuesday but is expected to be ready in mid-November. Covered California spokesman Ken Wood said small-business owners could still compare plans and get quotes starting Tuesday.