Thousands of Kaiser Permanente mental health professionals throughout California started a weeklong strike Monday to protest what they say is a lack of staffing that affects care.

Outside Kaiser Permanente hospitals and clinics in the San Francisco Bay Area, dozens of workers marched Monday holding signs that said: "Kaiser, Don't Deny My Patients Mental Health Care," and "Care Delayed is Care Denied."

About 4,000 psychologists, therapists, social workers, psychiatric nurses and other medical professionals represented by the National Union of Healthcare Workers say they will picket through Friday.

Some non-urgent mental health and other appointments may need to be rescheduled but anyone in need of urgent mental health or other health care will receive the services they need, said Elita Fielder, a spokeswoman for Kaiser Permanente.

"They've canceled appointments for these five days, but there's a critical situation every day of the year," Sal Rosselli, National Union of Healthcare Workers president, told the East Bay Times.

Rosselli said patients have to wait a month or more for follow-up appointments because of inadequate staffing.

Fielder said Kaiser Permanente has added more than 500 mental health care therapists and invested $175 million to expand mental health care offices since 2015, when the threat of an open-ended strike was averted after the union and Kaiser agreed to a contract.

The union's main concern is increasing its workers' wages, which she said are already the highest in the state.

Rosselli said negotiators are seeking pay increases as well as benefits packages equal to those given to other medical professionals.

Although Kaiser has hired hundreds more mental health professionals, patient care and access have stayed the same or worsened as the health care provider has expanded its client base significantly and some caregivers have left, Rosselli said.