Heath said it was hit-or-miss trying to get to the online application's "first user information page," where an account can be set up.

To reduce congestion, state officials turned off the "preview plan" feature on the website, said Peter Lee, executive director for the online insurance exchange. The website also asked consumers who started the enrollment process to return and finish their application later, when there is less traffic.

While midnight Monday was the deadline to apply for coverage, California officials had previously announced the state would allow people who started the process on time to complete their applications by April 15.

On Monday, the state expanded that pool to include people who were unable to create an online account or start their application Monday because of technical problems. However, these people must work with a certified delegate — an enrollment counselor, insurance agent, county eligibility worker or Covered California counselor — to complete their application and choose a plan by April 15.

"We had not planned for a fourfold increase from our highest day. Because of that we, adjusted our policy to meet consumers' needs," Lee said in a conference call with reporters.

Under the federal Affordable Care Act, most people are required to have insurance or face a tax penalty, which starts as little as $95 per year but increases over time.

Most Americans were not required to meet Monday's deadline. More than half of Americans under the age of 65 receive health care through employers, while 21 percent are covered by Medicaid or another form of government insurance, according to a 2012 study by Kaiser Family Foundation. Almost six percent purchase health insurance through the individual market. Nearly all Americans over 65 are covered by Medicare.

The law targets the remaining 18 percent of the U.S. population that is uninsured. Some are eligible for subsidized health insurance through Covered California and other exchanges, while others will be steered to Medicaid, the safety net program known as Medi-Cal in California.

California's experience was similar to that in many other states, where a crush of procrastinating consumers flooded call center phone lines and bogged down the online marketplaces. A software problem with the federal government website, which operates in 36 states but not in California, temporarily prevented new users from creating accounts.

On Monday, six enrollment specialists working in Sonoma County public health offices in downtown Santa Rosa reported sluggish performance on the Covered California website.

"The website is spotty, may work okay for a little while and then not," said Sandy Sakwa, a health coverage training specialist for the Redwood Community Health Coalition.

The process took four times longer than normal and in many cases information had to be entered repeatedly, Sakwa said.

Dan Kessler, another insurance broker certified by Covered California, spoke with several people who tried to create an account with Covered California on Monday, but the website kept freezing midway in the process. When that happened, he said, applicants triggered a form of online limbo.

"They can't log back in and create a new account, because the Covered CA website is recognizing their name and partial information, but now won't let them create a new account," Kessler said.

At one point Monday, Covered California representatives notified certified enrollment counselors that they should enter the minimum information into the system so clients could set up an appointment for a later date to complete the application process.