LOS ANGELES (AP) — A medical marijuana trade group is suing Los Angeles to stop the city from implementing a ban on prescription pot dispensaries.

The suit filed by Patient Care Alliance alleges the ban violates the constitutional right of freedom of assembly, the Los Angeles Times reported Friday.

The ordinance passed last month by the City Council said medical marijuana may be grown and shared only by groups of three people or fewer, and it outlaws the sale of the drug in stores.

Marc O'Hara, who heads the trade group, said the ban is "unconstitutional on its face" and said it denies patients their right to assemble and care for each other.

"We believe that this is a very necessary medical need in the city of LA," O'Hara said.

The city is notifying dispensary owners that they must shut down by Sept. 6, when the ban takes effect. In a letter sent out this week, city lawyers warned that those who don't comply face fines or jail time.

Special Assistant City Attorney Jane Usher told the newspaper the city had not yet seen the lawsuit and had no comment.

Patient Care Alliance works with more than 300 dispensaries in Los Angeles.

The suit comes as other groups of medical marijuana activists are working to qualify a ballot measure to repeal the ban, the Times reported. The Greater Los Angeles Collectives Alliance and Americans for Safe Access have joined forces with a labor union of dispensary workers to gather enough signatures to put the issue to voters.