Days after a foie gras ban came into force in California, a Los Angeles restaurant group and others have filed a lawsuit seeking to overturn the bill outlawing the controversial gastronomic delicacy.

Hot's Restaurant Group, Canada's Association des Eleveurs de Canards et d'Oies du Quebec and New York-based producer Hudson Valley Foie Gras claim the ban is "unconstitutional, vague and interferes with federal commerce laws."

California lawmakers agreed the ban in 2004, but gave the western US state's only foie gras producer seven and a half years to comply before it came into effect on July 1 this year.

Restaurants serving the gourmet item -- made from force-feeding ducks or geese -- can be fined up to $1,000.

But the legal challenge claims the 2004 law is unclear in defining what constitutes force-feeding, said attorney Michael Tenenbaum, who filed the lawsuit in Los Angeles this week.

"The Bird Feeding Law does not provide any intelligible measure -- such as weight, volume, or caloric value -- by which those involved in the feeding of the ducks ... may determine at what point a duck has been fed 'more food' than the statute allows," says the lawsuit, cited by the Los Angeles Times.

Tenenbaum said he is seeking a preliminary injunction to halt the law until the lawsuit -- which cites California governor Jerry Brown and Attorney General Kamala Harris -- goes to trial.

In the run-up to the July 1 ban, some of the Golden State's top chefs including Thomas Keller, the only US chef with two three Michelin-starred restaurants, redoubled efforts to persuade lawmakers to overturn the ban.

Calling themselves the Coalition for Humane and Ethical Farming Standards (CHEFS), they have staged a series of foie gras-rich evenings to raise money for the cause.

But John Burton, the former California legislator who drafted the law, dismissed their calls, likening the tradition of foie gras to waterboarding and female genital mutilation.

"I'd like to sit all 100 of them down and have duck and goose fat -- better yet, dry oatmeal -- shoved down their throats over and over and over again," he told the San Francisco Chronicle in April.