The suspect's identity has not been released by the Los Angeles Police Department, which announced the arrest in a tweet published Thursday.
The LAPD credited the arrest to "the trained eye of an LAPD Harbor Division officer," who initially detained the suspect.
A photo released by police appear to show a rifle, shotgun and a holstered handgun that were confiscated. Another photo shows several boxes of ammunition and gun magazines that were seized.
"I'm grateful for the women and men of the Harbor Division for their readiness and efforts to keep our community safe every day," Los Angeles City Councilman Tim McOsker, whose district includes San Pedro, said in a statement. "What could have been a horrific event was prevented because of smart, thoughtful police work."