Don’t get between an opera singer and her frozen, vegan pad Thai with tofu.

A fight between amateur mezzo soprano Marcella Caprario and fellow shopper Dr. Cathleen London in the frozen-food aisle of the Trader Joe’s on the Upper West Side is set to hit Manhattan Criminal Court today.

In one corner is Caprario, a 37-year-old mom who also teaches grade-school English.

In the other is London, a family practitioner, occasional TV talking head, avid triathlete and divorced mother of two boys who has appeared as an expert on “The Joy Behar Show,” CNN and Fox TV.

London says she was shopping in the popular grocery on Broadway at 72nd Street last winter when frozen-pad-Thai-dinner fan Caprario slapped her in the face, “causing redness, swelling and substantial pain,” according to court papers.

Caprario admits that she slapped the doctor — but insists that the doctor needed slapping.

Caprario’s husband, Bill, was reaching on her behalf for the frozen pad Thai and the doctor’s son got in Bill’s way, the singer insists.

“Not that we’re condoning slapping people, but this was justified, reasonable force,” said Caprario’s lawyer, Mark Bederow.

Caprario and her lawyer said the incident unfolded on the second Sunday afternoon in January, as Caprario’s husband was making a grab for the Trader Joe’s brand Frozen Vegan Pad Thai With Tofu — his wife’s favorite.

“You know how they have those awful pillars outside the frozen-food cases?” Caprario told The Post yesterday. “Bill’s there, trying to reach around them” to get to the pad Thai.

That’s when London’s older son, a teenager, seemingly intentionally kept trying to get between Bill and the ethnic treat — just to be obnoxious, said Caprario and her lawyer.

“So Bill says to Marcella, loud enough for the kid to hear, ‘They don’t even say excuse me,’ ” Bederow said.

London hit the roof, telling Caprario, among other things, “He’s just a child! Get that pole out of your ass!” Caprario said.

Caprario and her husband may take the stand as early as today to describe the resulting verbal dispute — and the slap Caprario admits giving London when the doctor “charged at” her, “getting into her personal space.”

Caprario had originally been charged with misdemeanor assault and has rejected various plea deals as prosecutors repeatedly lowered the charges — first to harassment and then to attempted assault.

“Why should someone plead guilty to something they didn’t do?” she said.

London, who practices at the Iris Cantor Women’s Health Center on the Upper East Side, declined to comment when reached at her home yesterday.

Additional reporting by Jessica Simeone

laura.italiano@nypost.com

