Thousands of unionized Kaiser Permanente therapists and other mental health workers began a planned five-day statewide strike Monday, with hundreds marching outside the health system’s San Francisco medical offices to protest long wait times for patients seeking appointments for depression, anxiety, bipolar disorder and other mental health conditions.

Most striking employees are members of the National Union of Healthcare Workers, which represents psychologists, therapists and clinical social workers, and is in contract negotiations with Kaiser. Some Kaiser nurses, members of the California Nurses Association, joined in solidarity.

Kaiser facilities remain open during the strike. Patients who have appointments for mental health services scheduled for this week are being seen by managers or psychiatrists, or have moved their appointments to another week.

Workers plan to protest outside Kaiser facilities across the state every day this week, including on Tuesday in Oakland, Santa Rosa, Santa Clara and elsewhere.

Oakland-based Kaiser is the nation’s largest integrated health system with about 12 million patients in the United States, including 4 million in Northern California.

The workers’ primary concern is that many patients are having to wait four to six weeks before they can see their therapist or psychologist for follow-ups after their initial appointments. They say Kaiser should hire more therapists to shorten the wait time, which can be critical when treating mental health disorders.

“They’re desperate and it takes too long to get them in,” said Alia Prince, a licensed clinical social worker at Kaiser in Santa Rosa who works with patients with postpartum depression. “They need help and we need to get them help,” Prince said. “They sometimes wait a month (for a follow-up appointment). Ideally, it’d be within the week.”

Michelle Gaskill-Hames, a Kaiser senior vice president, released a statement saying the health system has increased staff by 30 percent, hiring 500 new therapists in California since 2015, and invested $175 million to improve mental health care services.

In 2013, Kaiser was fined $4 million by the California Department of Managed Health Care for inadequate patient access to mental health treatment, one of the state’s largest insurer fines. Last year, the agency again criticized Kaiser for delays in behavioral health treatment, but stopped short of issuing another fine.

“We apologize to our patients and members for any disruption,” Gaskill-Hames said. “We really feel this is unnecessary.”

Gaskill-Hames said the union is seeking higher wages and benefits even though Kaiser is the highest paying employer for mental health workers in California. In Northern California, most Kaiser psychologists and social workers earn at least $138,000 and $111,000, she said, slightly more than their counterparts in Southern California.

The National Union of Healthcare Workers’ last strike, in 2015, was also over a contract dispute. Contract talks will resume next week, a union spokesman said.

Catherine Ho is a San Francisco Chronicle staff writer. Email: cho@sfchronicle.com Twitter: @Cat_Ho