MINNEAPOLIS - More than 12,000 nurses launched a one-day strike Thursday at 14 Minnesota hospitals in a dispute over nurse-patient ratios and pension benefits.

Nurses say that they are being asked to care for too many patients at a time and that strict ratios are necessary to protect patient safety. The hospitals, all in the Minneapolis area, counter that patients are safe and that the walkout is a headline-grabbing stunt to build membership and clout for a national nurses union.

About the same number of nurses had planned a simultaneous strike in California over the same issues but were blocked temporarily earlier this week by a San Francisco judge.

The Minnesota nurses walked off the job at 7 a.m. and onto picket lines at several sites. At Abbott Northwestern Hospital, one nurse serenaded several hundred others by playing "Amazing Grace" on bagpipes. Passing motorists honked horns, and red-T-shirted nurses waved signs that read, "We care. For you" and "RNs protecting patients."

Sue Stamness, a cardiology nurse at Abbott for 24 years, said patient safety was the nurses' top concern.

"Nobody is listening to what we are saying," she said.

Though called the largest nurses' strike in U.S. history by both the union and the hospitals, the immediate effect was expected to be minimal. Hospitals hired 2,800 replacement nurses, called in extra nonunionized staff, reduced patient levels, and some hospitals rescheduled elective surgeries. Staff of two of the metro area's largest hospitals were not involved in the strike.

Near midday, hospital officials said they were having no problems with patient care, with more than enough nurses on hand. Maureen Schriner, a spokeswoman for the hospitals, said that all hospitals were open and that emergency and childbirth departments were fully staffed.

The Minnesota and California negotiations are the largest since the National Nurses United union formed in December, combining the California Nurses Association, the Massachusetts Nurses Association, and the United American Nurses, which was primarily Midwestern nurses.

Like other businesses, hospitals are trying to trim their budgets even as health-care costs have been skyrocketing. Nurse pay and benefits are among the hospitals' largest expenses.