LASD officials said Downey police confirmed there were no injuries and the building is secured.
Authorities arrested 34-year-old Jesus Chavez of Lynwood. A handgun was recovered at the scene, but detectives investigating determined no shots were fired inside the building.
Chavez was booked for criminal threats and transported to Los Angeles County Jail.
At an afternoon press conference, authorities said the suspect caused a disturbance at the hospital and threw a chair out of a window before producing a handgun.
Police said evidence indicates Chavez acted alone. The investigation is ongoing.
The incident started shortly before noon, and LASD Norwalk Station initially tweeted that they were assisting Downey police with a "fluid," emergency situation and "possible active shooter" at the medical center, located in at 9333 Imperial Highway. Within minutes, sheriff's officials tweeted that a suspect was detained at the scene.
Officers and deputies searched the hospital "room by room, floor by floor" for more than an hour while hundreds of people evacuated the medical center. Much of the activity was focused on Building A, officials said.
Many witnesses said it all happened when a patient in the mental health unit started kicking walls and breaking windows. After hearing that the man had a weapon, some people sheltered in place, while others fled the building.
Betty Esquival is a patient who was in a hospital room when she heard the commotion start.
"We started hearing screaming and yelling outside...he was saying, 'I have a gun, people are going to die,'" she described.
Witness Amber Boughner said the man appeared to be angry over a doctor's appointment.
"He started pounding on the door, saying, 'I had an appointment at 11 o'clock! I deserve my appointment, where is that B.I.T.C.H. Make her let me in!' And he just kept pounding, pounding and pounding," Boughner said.
She said she saw the man kick walls and throw furniture out of a window until security arrived. Broughman said when the security guard confronted the man, he told her the suspect was armed and she needed to leave.
"The security guard came out and he said, 'He's got a gun, run,'" Boughner added.
She then locked herself with a few other inside a break room until authorities allowed them to get out.
"I was seriously scared for all the employees that were in there," she said. "He was angry, he was very angry."
Downey PD Chief Carl Charles said the man surrendered to authorities as soon as he was confronted.
"They immediately confronted the subject, he surrendered, and he gave himself up without incident. He was arrested and taken into custody," he said.
Authorities said there was active shooter training at the center back in April between police and security and believe that helped get the situation under control.
Officials said they were investigating whether the sound of furniture being thrown out the window may have been mistaken for gunshots reportedly heard by people inside the building.
Some buildings will remain closed for several hours as the investigation continued.
All appointments at the facility were canceled for the rest of the day and services are being offered from Kaiser Permanente to anyone affected by the incident.
The company issued a statement saying it's grateful there were no injuries.
"The hospital, emergency and urgent care departments, and most of the Orchard Medical Office Building at Kaiser Permanente Downey Medical Center have reopened. We are working to reschedule all missed appointments that were canceled due to this incident.
The safety of our patients, employees and physicians are paramount, and we're grateful there were no injuries. We recognize the stress that can result from situations like this and are offering behavioral health services onsite to members, employees and physicians.
We want to thank the Downey Police Department and the Los Angeles County Sheriff's Department for their extraordinary response and support."