The move comes amid a widening dispute over pot shops in California's most populous city. Residents complain dispensaries are a nuisance that draw riffraff, but the store owners say they serve patients with serious diseases like cancer and AIDS.

"We believe that this total ban would not allow for safe access for some of the city's sickest patients," said Rigoberto Valdez, vice president of a local union and a member of the Committee to Protect Patients and Neighborhoods.

The committee said it would turn in 50,000 signatures on Thursday, which would immediately put the ban on hold while officials check if more than 27,000 valid signatures are there to force a March 2013 referendum. Until that vote, the city would be prohibited from enforcing the ban, officials said.

Los Angeles has an estimated 1,000 dispensaries. They have multiplied quickly in recent years, even though pot remains illegal under federal law and the U.S. government has cracked down on several cannabis shops in southern California.

A 14-0 vote last month by the mostly liberal Los Angeles City Council to close down the medical marijuana dispensaries represented a significant blow to the industry in a state where voters in 1996 became the first to allow the drug as medicine. But officials had expected the industry to fight back.

The committee challenging the ban is made up of organized labor for dispensary employees, the shops themselves and patients, said Valdez, vice president of United Food and Commercial Workers local 770. The group supports reducing the number of dispensaries in the city to 100.

Seventeen states and the District of Columbia allow medical marijuana. Voters in Washington state, Colorado and Oregon will decide in November whether to allow recreational use of pot.

Los Angeles City Councilman Jose Huizar, the author of the ordinance to ban dispensaries, said he proposed it due to nuisance complaints from residents and dispensaries selling to recreational users faking a medical ailment.

"I've got parents complaining to me that they're walking with their kids down the street, and there's marijuana smoke either coming from the dispensary or people smoking it on the street outside," Huizar said.

Medical marijuana trade group Patient Care Alliance and 11 patients filed a lawsuit in Los Angeles Superior Court this month seeking to block enforcement of the ban. A judge has not ruled on that request from the group, which is separate from the activists planning to submit signatures on Thursday.

(Reporting By Alex Dobuzinskis; Editing by Cynthia Johnston and David Brunnstrom)

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