By Brandon Lowrey

LOS ANGELES (Reuters) - The animal rights group PETA sued a California restaurant on Wednesday that it says serves outlawed foie gras to patrons, in what appears to be the first lawsuit to enforce a state ban on the delicacy, PETA officials said.

People for the Ethical Treatment of Animals said that Hot's Kitchen claimed to have stopped selling foie gras when a ban on producing or selling it took effect in July, but was offering it as a complimentary side dish to customers who order "THE Burger."

"It's a transparent attempt to evade the law, plain and simple," said Jeff Kerr, general counsel for PETA. He said the suit was filed on Wednesday in Los Angeles Superior Court.

The Hermosa Beach restaurant's online menu lists "THE Burger" as including balsamic thyme onions and whole grain mustard, and says it is served "with a complimentary side of foie gras."

Foie gras is the swollen liver of a duck or goose that has been excessively force fed. It is prized for its flavor and rich texture, but animal rights groups contend that the force-feeding process is painful, gruesome and inhumane.

California lawmakers banned the sale or production of foie gras in 2004 under penalty of a $1,000 fine per sale, per day. The law gave restaurants and farmers a years-long grace period that ended July 1 to implement the ban.

Most restaurants removed the item from their menus when the law took effect. However, some reportedly left foie gras on the menu after the ban.

In a widely reported act of rebellion, the Presidio Social Club in San Francisco served foie gras sliders to a crowd as protesters amassed outside. The owners contended the social club was not subject to the state law because it was on federal land, but the restaurant later pulled foie gras from its menu.

Hot's Restaurant Group, of which Hot's Kitchen is a part, has been at the forefront of a charge to overturn the law, and sued California in federal court the day after the law went into effect, calling it unconstitutionally vague.

"The statute defines 'force feeding' as using a process that causes a bird to 'consume more food than a typical bird of the same species would consume voluntarily,'" according to the legal complaint filed by Hot's Restaurant Group and a Canadian non-profit.

"In practice, the vagueness of this purported standard makes it impossible for anyone to know at what point a particular bird has been fed 'more food' than the bird feeding law allows."

The restaurant's spokeswoman, Kelley Coughlan, said the restaurant had so far not been served with the suit.

"Publicity stunts such as the filing of an outrageous, baseless lawsuit, followed by the issuance of press releases are nothing more than an attempt to exploit the media by stoking controversial flames and are designed to line the pockets of profiteers," she said.

"Hot's stands by its previous statement that foie gras can be made humanely, and we continue to provide our customers with wholesome, humane animal products."

California Attorney General Kamala Harris is defending the foie gras ban, according to spokeswoman Lydia Gledhill.

Gledhill said she was unaware of any legal action against purveyors of enlarged bird liver since July, but that the state wasn't formally tracking the law's enforcement.

(Editing by Cynthia Johnston and Lisa Shumaker)