Bridgehampton Polo’s posh opening on Saturday was marred by an altercation over a glass of spilled rosé — during which Upper East Side restaurateur Nello Balan was accused of calling another guest “a Pakistani piece of s – – t !”

The trouble began at the supposedly sophisticated affair — where spectators included Christie Brinkley and Donna Karan — when celebrity dentist Dr. Shawn Sadri was jostled and dropped a glass of wine on the grass.

Sadri said “only five or 10 seconds” passed when Balan, whom he’d never met, came racing across the VIP tent from “7 feet away,” and yelled, according to Sadri, “Are you going to pick up that glass? There are kids running here, you [bleeping] Pakistani piece of s – – t!”

Sadri told us he asked Balan, “Where would you like me to put it?” and Balan bellowed, “You’ll see what I do to you!” The Persian dentist, who moved to Riverdale with his family when he was 3, related: “I said, ‘Go ahead. What are you going to do?’ ” — and Balan backed down and “went back to sulk in a chair.”

“I don’t let things like that bother me,” Sadri said. “It was inappropriate. People gasped when out of nowhere, there was this racial epithet.”

But Balan disputed Sardi’s account. He said he was concerned because a child was running barefoot nearby. “The guy threw his glass on the floor,” Balan said. “Here was a little, 3-year-old girl running around in bare feet. I said, ‘Excuse me, can you pick up the glass?’ He refused. I was in shock. He asked, ‘Where should I put it?’ And I said, ‘Put it up your ass!’ ”

As far as the alleged ethnic slur, Balan said, “I did not call him a piece of s – – t. He called me a ‘Russian mobster.’ I never called him Pakistani. I said, ‘If I’m a Russian mobster, what are you?’ He apologized and gave me his business card.”

Sadri, who’s treated clients including Tinsley Mortimer and DJ Afrojack at his Manhattan practice with Dr. Laura Torrado, told us, “I didn’t even know who he was.” A guest said the situation was “more ‘Real Housewives’ than Bridgehampton Polo.”