The controversial ordinance - approved as Proposition L by 54 percent of voters in November - makes it illegal to sit or lie on public sidewalks between 7 a.m. and 11 p.m.
The law has exceptions for medical emergencies, disabled persons using wheelchairs, parades and protests, lawful sidewalk businesses, or waiting in a line. It also requires that police give an initial warning before citing violators.
A first citation could bring a fine between $50 and $100 and/or community service. Multiple convictions in a 24-hour period could bring fines between $300 and $500 and up to 10 days in county jail.
Though the law became effective today, police said in a statement that it is still developing training and enforcement policies, as well as a public education campaign that will run in January.
The training for officers is "to ensure that they fully understand the ordinance and enforce it fairly," the Police Department said in a statement. "The Department is committed to dedicating its resources to ensure that officers know the limits and requirements of the law and are able to serve as a resource for the public."
November's ballot measure was pushed by local business owners, particularly in the Haight-Ashbury neighborhood, who complained that people walking past their shops were getting harassed by aggressive street youth.
Police also supported the measure, saying current law provided inadequate tools to enforce against such behavior.
Homeless advocates condemned the proposal as an attack on all homeless people.
Similar measures have cropped up in other cities, including Santa Cruz, Berkeley and Palo Alto in the Bay Area.