You would probably need a permit, but --
may I suggest that the occupiers should march past the private clubs -- the Jonathan Club, the Los Angeles Athletic Club, the City Club and the highly elite club around the corner from the downtown library:
Info on the Jonathan Club including the downtown address.
http://www.google.com/search?q=jonathan%20club&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&source=hpchannel=nphttp://www.clubcorp.com/Clubs/City-Club-on-Bunker-HillThe City Club is on an upper story of a building if I recall correctly, but anyway:
http://www.clubcorp.com/Clubs/City-Club-on-Bunker-HillHere is info for the Los Angeles Athletic Club.
http://www.google.com/search?q=los%20angeles%20athletic%20club&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&source=hp&channel=npAnd, in my experience, the most elite of them all: The California Club
http://www.yelp.com/biz/california-club-los-angelesThere is nothing wrong with the clubs or the people in them, but that is where the influential and upper middle-class to wealthy meet, socialize, and where you will most likely be seen and heard by them.
There is little point in sticking around City Hall on a weekend.
There is a lovely park right next to the library, and the swanky private clubs are quite close. You probably can't go in for lunch, but you might be able to walk by and say Hello to their members and guests.
City Hall is not where it's at in my opinion. You want to be near the big law firms and these clubs. Limiting the march to downtown LA, that's the closest thing to Wall Street.
As I said, you probably need a permit for this.