The newly-released footage was captured June 20 after an employee at a McDonald's on Indiana Avenue called 911, according to the Riverside Police Department.
The man, identified by police as Peter Villalobos, had reportedly locked himself inside the restroom and would not respond for more than an hour.
Officers eventually got the man to exit the bathroom, but that's when they say he became combative and refused to follow commands.
"They're trying to put him in handcuffs, and the man resisted, and they had to use a level of force," said Officer Ryan Railsback.
Video shows two officers pushing the man's head into the hood of the police car as they tried to detain him. Both of them repeatedly punched the man, eventually causing him to fall over.
"When someone is resisting that arrest, we have a duty to overcome that resistance as well," Railsback added.
It's hard to tell what happened next, but from security video, officers can be seen on top of Villalobos. After the altercation, the man can be seen on the ground, panting and saying he was in pain.
"I just feel so bad because I wasn't there. He would never, ever do anything to anybody," said his mother, Olivia Villalobos. "What they did to my son, he didn't need that... He just wanted to come home."
Riverside police say Villalobos was taken to the hospital. A few hours later, he suffered a medical emergency and was pronounced dead.
Police said at the time of the incident, Villalobos had an outstanding warrant for being under the influence of a controlled substance.
His mother says he was diagnosed with schizophrenia 10 years ago.
"What they did to my son, he didn't need that... He just wanted to come home," she told Eyewitness News.
An autopsy is being conducted to determine the official cause of death. An outside agency is investigating the Riverside Police Department's use of force.