"Surgeries are continuing, babies are continuing to be born," hospital spokeswoman Karen Garner said.

A strike involving 23,000 nurses would be large, particularly in recent years when the strength of organized labor has declined, said John-Paul Ferguson, assistant professor of organizational labor at Stanford University.

"It's rare for us to have these days in the United States that many people on a coordinated strike," Ferguson said.

The eight hospitals employ about 4,500 nurses who are members of the union, according to Garner. At least at two of the hospitals, a majority of unionized nurses who were scheduled to work reported for duty, Garner said.

"Many of our nurses recognize that our hospitals provide competitive wages, competitive benefits and a rewarding working environment, and they've chosen to report to work to care for our patients," she said.

Children's Hospital Oakland was also expected to bring in replacement nurses. The hospital had rescheduled elective surgeries as well, according to Erin Goldsmith, a hospital spokeswoman.

Kaiser officials said many nurses had crossed the picket line and their hospitals were "fully staffed."

The focus of the strike was Sutter Health, where union officials say nurses in contract negotiations are being asked to accept thousands of dollars in higher costs for their health care.

Sutter's proposed concessions would also reduce the ability of certain nurses to advocate for patients, cut pay for newly hired nurses, and slash vacations and holiday pay, according to the union.

At Alta Bates Summit Medical Center in Oakland — a Sutter Health-affiliated hospital — nurses held signs that read, "Community Care, Not Corporate Profits," and "Some Cuts Don't Heal."

"We're not just here for money," said Vicki Theocharis, a nurse in the hospital's oncology unit. "We want to take care of the patients, and it almost feels like we're being penalized for doing it."

Millicent Morland, 65, a nurse on the union bargaining team at Summit Medical Center, said the cuts management has proposed would set a dangerous precedent in the health care industry.

"All other management will think about it and pass it on to workers," she said.

Garner has said the hospital has an obligation to keep health care costs down for patients.

Union members at Children's Hospital, who have been without a contract for more than a year, have objected to a proposed increase in the cost of a health care plan the hospital offers.

At Kaiser Permanente, 17,000 nurses were expected to walk-off the job to show solidarity with the National Union of Healthcare Workers. The union is in contract talks with Kaiser and facing demands for cuts in health and retirement benefits, Idelson said.

Kaiser Permanente has said it is bargaining in good faith.

The strike is scheduled for one day, but Sutter Health and Children's Hospital officials said nurses will not be able to immediately return to work because the hospitals' contracts with replacement agencies require a minimum number of days of service.

At Sutter Health, that ranges from three to five days, Garner said. At Children's Hospital, it is five days, according to Goldsmith.

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Associated Press writer Terence Chea contributed to this report from Oakland.