The Los Angeles City Council's effort to close down the city's medical marijuana dispensaries next week could face a serious challenge Wednesday, when activists say they will submit a petition with 50,000 signatures to overturn a recently approved ban.

The petition that will be turned over to the L.A. City Clerk's Office calls for a referendum next March on the new ordinance banning storefront dispensaries effective Sept. 6. But the petition's immediate effect will be to prevent the ordinance from even going into effect.