WASHINGTON—The National Rifle Association is asking Supreme Court Justice Anthony Kennedy to block enforcement of a California city ordinance that bans gun magazines capable of holding more than 10 bullets.

The ban in Sunnyvale went into effect Thursday after 66 percent of voters approved it in November. The NRA challenged the law, but a federal judge ruled last week that it does not violate the Second Amendment right to own guns for self-defense.

The 9th U.S. Circuit Court of Appeals in San Francisco denied the NRA's request for an emergency order to stop the ban from taking effect pending appeal.

The new ordinance allows gun owners to have police destroy the magazines, sell them out of state or to a licensed gun dealer, or move them out of town.