LOS ANGELES—Medical marijuana advocates are gathering signatures for a voter referendum to block a recently passed Los Angeles law that will shutter hundreds of pot shops.

The effort is designed to overturn the city's medical marijuana dispensary ordinance before it takes effect in May.

Monday is the deadline to turn in 27,425 signatures and organizer Dan Halbert says "it's going to be close."

Halbert's dispensary opened last year and is one of hundreds that would have to close under the ordinance, which caps the number of pot dispensaries at 70. The law makes an exception for about 128 dispensaries that registered in 2007, when the City Council adopted a moratorium.

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