(Reuters) — California's Obamacare insurance marketplace experienced a surge in enrollment last week, officials said on Thursday, offering welcome news for President Barack Obama's sweeping health care law from a state crucial to its success.

Some 144,146 applications were completed and 49,708 people selected commercial plans in the first week of December, compared with 403,323 completed applications and 109,296 enrollments in the previous two months since the Covered California marketplace opened, officials said.

At that rate, California would meet its 2013 enrollment target.

"This is a good day for Californians," Peter Lee, executive director of Covered California, the state's Obamacare marketplace, told reporters. "We are seeing real momentum."

California is arguably the most crucial state for Obamacare. It has more uninsured people than any other state (7.4 million in 2011), and the law's supporters are counting on Californians to make up a good fraction of the 7 million people the White House hopes to enroll in health insurance through the law during this first open enrollment period, which runs through March 31.

On Wednesday, federal officials announced that 365,000 people nationwide had chosen a health insurance plan through their state's Obamacare exchange through the end of November. California accounted for 109,276 of those, Mr. Lee said.

Altogether, the 50 states and the District of Columbia had reached only 30% of their combined enrollment targets through Nov. 30, in large part because of disastrous technical problems with the federal website, Healthcare.gov, through which people in 36 states access Obamacare plans. California's website has not suffered those technical problems.

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The enrollment momentum the state saw in the first week of December grew this week, Mr. Lee said, with about 31,000 Californians choosing a commercial plan on Dec. 8 and 9.

"Enrollments and applications are surging, and we at Covered California — and our partners — are stepping up our game to meet the demand," Mr. Lee said.

In advance of the Dec. 23 deadline for enrolling in a plan for coverage to begin on Jan. 1, he said, enrollment workers will be on the job the next two Sundays and the state will institute a speeded-up way to enroll people by paper if they cannot do so on the website.

In another hopeful sign for the law's supporters, 22% of California's October and November enrollees were ages 18 to 34, an age group that constitutes about 25% of the state's population.

Health economists and insurance companies have warned that unless significant numbers of young adults enroll in the commercial plans being sold under the Affordable Care Act, the plans could be financially unsustainable.

In a more worrisome trend, Californians ages 45 to 64 accounted for 45,945 enrollees, or 58% of the total. They make up 25% of the population. There is concern that if older adults make up too much of the insurance pool, their often-higher medical claims will also threaten the plans.

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In addition to the Californians who have chosen a commercial insurance plan, since October 179,000 people were determined to be likely eligible for Medi-Cal, the state's Medicaid program for low-income people. Medi-Cal is expected to enroll 1 million to 2 million more people by next March, Mr. Lee said; before Obamacare enrollment began, it covered 8.5 million.

One disappointment is that Latinos are enrolling in Obamacare at rates much lower than their proportion of the state's population. About 29% of Californians are Spanish speakers, but they make up only 4.6% of enrollees so far.

"That's not as high as we want" and "an area where we need to do more," said Mr. Lee, adding that Covered California will conduct an review of its marketing to see why the program is not attracting more Latinos.