Brandon Piddington, a reporter online news service Axcess News recorded part of the incident with an iPhone that showed one of the police officers repeatedly hitting Johnson with a baton while another officer holds him down.
"My first reaction was to not film it," he said.
But Piddington quickly changed his mind and pulled out his iPhone.
"This guy was really getting hammered on, and I thought it was wrong."
On the tape, bystanders can be heard saying, "That's too much," and "That's enough."
The San Bernardino Police Department said the incident is under investigation.