California Code, Government Code - GOV § 37359

Unless otherwise provided by law, the legislative body having control of any property owned or controlled by the city may at any time withdraw the property from the personal access and use of members of the public, or limit the access or use in area or time or in any other reasonable manner deemed necessary.  Any person thereafter using the property without permission or in a manner other than that prescribed is a trespasser.  This section does not limit or restrict any person from access or use who has a private right in the property.