More than 2,400 Kaiser Permanente psychologists, therapists, social workers and other mental health workers in Southern California began an open-ended strike Monday over increased workloads and staffing shortages that their union said have created a “substandard” system of care.

The National Union of Healthcare Workers said that nearly 2,400 mental health workers had launched the job action after Kaiser Permanente management turned down their proposals.

The union that represents nearly 2,400 mental health care workers in Southern California — 316 in San Diego County — filed a strike notice with Kaiser Permanente on Friday, putting their employer on notice that they intend to launch an “indefinite” strike starting on Oct. 21.