As I understand it, you don't need a license or permit to open carry, with a loaded magazine on your belt.
You cannot really "open carry" even an unloaded gun in California on your travels during a typical day. Open carry of even an unloaded firearm is for all intents and purposes banned. This is due to the California Gun Free School Zone Act. The inspiration for the federal version, but unlike the federal version it is quite active.
This means you can open carry if you never go within 1,000 feet of a school, which is wider than virtually all roads.
It is nearly impossible to go through a town without going within 1,000 feet of a school during regular travel.
You would need a route that was a zigzag pattern and well planned out to accomplish this.
Even many rural areas have elementary, middle, and high schools at some point along or near most main roads.
This means you can certainly do it if you spend some time to research specific routes, or for political reasons in places that meet the criteria. But you won't be strapping even an unloaded firearm to your side and going about your regular day. Your typical urban and suburban area has dozens of schools dotting the landscape within 1,000 feet of main roads, highways, and freeways. By the time you will see a well marked public school or notice it you will also normally be within 1,000 feet of its property. You also cannot realistically stop and turn around in traffic if one is coming up in 1,200 feet as you come around a corner and notice it.
So no, open carry even open unloaded carry is not really legal per say. It is but it isn't. You would be arrested traveling in a straight line through almost any urban or suburban area, and breaking the law even in most rural locations.
It is impossible without researching all local schools and plotting a strange course to legally do it.
The groups that do it for political reasons research the location and plan to do it beforehand. They insure it is far from prohibited locations, and take extensive steps to break no laws while doing something that attracts that controversial attention. They get to a location where they can put on their unloaded guns, hang out for a set amount of time, and then take them off before traveling.
Here is a good resource for locating all schools:
http://www.cde.ca.gov/re/sd/
For example
in just the city of Los Angeles, nevermind the county this lists
23 pages of public schools and 8 Pages of private schools.
In the County of Los Angeles it lists 93 pages of public schools and 36 pages of private schools
About 25 schools to a page!
That would give
close to 2325 of just public primary schools and 900 private schools to avoid by at least 1,000 feet at all times.
For a
grand total of about 3,225 places you may never drive or walk within 1,000 feet of in the county. Some schools are small and some quite large. So it can be within 1,000 feet of a mile or two long high school.
Simply unrealistic without extensive planning of specific streets. Then what do you do when a street is closed due to a water main burst, flood, debris, or car accident and the closure re-routes you? Most likely break the law.