There was a great deal of champagne spilled. It spilled from open bottles, poured by tuxedoed waiters into pyramids of oversize coupes. It spilled into the mouths of celebrities like Hugh Dancy, Claire Danes and Allen Leech as they gathered at Tavern on the Green on Sunday to celebrate the film “Downton Abbey: A New Era,” which premiered earlier that night at the Metropolitan Opera.

Unfortunately for some, the champagne also spilled onto dresses and suits as the oversize coupes were ill suited for the tipsy affair.

“This isn’t a coupe, it’s a bowl,” said one guest, dabbing her mauve Dior skirt with a napkin. “The dowager would not approve,” she said, referring to Violet Crawley, the matriarch of Downton Abbey played by Maggie Smith, who did not attend.

The champagne was not the only nod to grandeur. The restaurant was transformed into a villa on the Côte d’Azur, the film’s setting. Strings of lights, each with their own beaded chandelier, illuminated the courtyard, while glorious bouquets of roses and peonies exploded from urns. Buffets, tucked in corners, offered an array of seafood, hot and cold, including oysters Rockefeller, shrimp cocktail, clams casino and lobster.

“This is beautiful,” said Hugh Bonneville, who plays the family’s patriarch, Robert Crawley. He wore a suit from Cad and the Dandy, a Saville Row tailor. “I know it’s an iconic venue for New Yorkers, and it’s risen from the ashes like a phoenix.”

Fran Drescher, who lives nearby, is a regular. “This patio is just unstoppable,” she said. “Whenever I come here, I feel like I’m on vacation.”

Partygoers in semiformal attire (colorful dresses, suits without ties) clustered in the courtyard, congregating around tables reserved for cast members, the most popular belonging to actress Michelle Dockery, who plays Mary Crawley and wore a silver sequined Givenchy gown; and Julian Fellowes, the “Downton Abbey” creator.

“I love this place because this is where they had the after-party of a film called ‘Gosford Park,’ which I wrote 20 years ago,” Mr. Fellowes said. “And then it was the beginning of things for me, and I won the Oscar and all that stuff.”

Cast members from Mr. Fellowes’s other period drama, “The Gilded Age,” including Christine Baranski and Cynthia Nixon, mixed with the cast.

“I would watch ‘Downton Abbey’ and be this rabid fan and envy all those marvelous English actors,” Ms. Baranski said. “And now I’m part of a continuum. It’s like I’m part of an English American repertory company.”

The crowd thinned mightily after midnight, but Ms. Dockery and her table — including Laura Carmichael and Brett Gelman, in Bode — were still there, dancing under a cluster of palms to Luther Vandross’s “Never Too Much.”

Mr. Fellowes stuck around, too. “Well, I like to get the audience’s reaction,” he said.