Los Angeles is home to hundreds of medical marijuana dispensaries, but concern over their proliferation has provoked a backlash. Police are cracking down on most of them starting Monday.

A new city ordinance limits the number and locations of medical marijuana dispensaries allowed to operate in Los Angeles. Those that registered with the city before a 2007 moratorium may be able to remain in business. But they can't be near schools, libraries, parks and other sensitive areas. Police officers will begin closing down 400 unregistered dispensaries now operating illegally.

"The sky isn't going to fall down," says Asha Greenberg, assistant city attorney. "LAPD isn't going to go around kicking down doors, etc. Initially we're going to be doing information gathering."

Greenberg says L.A.'s new ordinance makes it a misdemeanor to run a dispensary without city approval.

"Anyone who is operating a medical marijuana establishment, who is violating the city's ordinance is subject to arrest," Greenberg says.

Dispensary owners and patients have filed more than 20 lawsuits against the city, arguing that the ordinance is unconstitutional because it prohibits access to their medicine. So far, their attempts at temporary restraining orders have been turned down in court.