Police approached the man and moments later one of the officers opened fire on the man.
"Within a matter of seconds, they saw him, they deployed on him," said LAPD Cmdr. Blake Chow. "They ordered him to drop the knife. He failed to do so, and then we had an officer-involved shooting."
Investigators are interviewing the witnesses who flagged down the officers for help.
The officers were not hurt during the incident.
"The officers are OK," Chow said. "They've been transported back to the station."
The man has not yet been identified.
One witness, who did not want to be identified, said the officer did not have to shoot the suspect.
Detectives will also be looking through surveillance tape to see if the incident was caught on camera.