Everybody rise!

This city was not about to let Stephen Sondheim’s 75th birthday pass unnoticed, and Saturday it threw him an exhilarating 12-hour party.

Loyal admirers of the composer-lyricist whose body of work has broken new musical-theater ground for half a century began lining up before dawn for “Wall to Wall Sondheim,” a free marathon program featuring a who’s who of Sondheim associates and performers from his shows.

By 11 a.m., when the festivities opened with a mayoral proclamation, Symphony Space -- a 760-seat former movie theater on Manhattan’s Upper West Side -- was crammed, and lines would continue to stretch down the block throughout the afternoon and evening.

Sondheim himself, looking casual and relaxed, was present from the start, and he warmly applauded the well-chosen program opener: an ensemble of talented youngsters vigorously performing a portion of “Into the Woods Junior,” a scaled-down version of 1987’s “Into the Woods” that he shaped specifically for young people.

What followed was a comprehensive history of his output, from songs he wrote as a teenager under the guidance of Oscar Hammerstein II, through his early contributions as a lyricist (“Gypsy”) while he waited for his opportunity to write both music and lyrics, through his incomparable Broadway catalog, from “A Funny Thing Happened on the Way to the Forum” to “Passion.”

This was a passionate, in-the-know audience; original cast members from the shows received hero’s welcomes. Cheers greeted such stalwarts as Chip Zien (“Into the Woods”) and Lonny Price (“Merrily We Roll Along”), but the acclaim reached stratospheric levels for two grandes dames: Elaine Stritch and Angela Lansbury.

Each took to the stage to a standing ovation. The ever-wry Stritch, her sleek legs still a marvel at 80, squeezed every drop of merciless disdain out of the classic “The Ladies Who Lunch,” which, she noted, Sondheim “wrote accidentally for me” in 1970 for “Company.” The audience roared with anticipatory delight at her opening line, “I’d like to propose a toast,” and needed no prompting as she reached the full frenzy of her concluding refrain: “Everybody rise!”

Lansbury, looking incongruously elegant to be reprising her performance as the earthy cockney Mrs. Lovett in “Sweeney Todd,” was joined by George Hearn, who has often played the title role of a murderous barber in the 1979 show. They could barely contain their delight as they sang the wickedly hilarious “A Little Priest,” savoring the prospects of their cannibalistic business opportunities. (The theater’s adjacent cafe cheerfully featured “Mrs. Lovett’s meat pies” on its menu, with various patties labeled “lawyer,” “clerk,” etc.)

Even these veterans had a moment of confusion with their lyrics -- a not-uncommon occurrence all day. As one song followed another, one could marvel not only at the exceptionally deft wit and wordplay of Sondheim’s songs but at the challenges they pose.

“A lot of people here today are panicking about their lyrics,” acknowledged Gregg Edelman, a frequent Sondheim performer who was in “Passion” and “Into the Woods” on Broadway and performed three numbers Saturday. “But if you’re open enough to what his words say, they take you on a journey, and you don’t have to force the journey to happen. He writes great characters who have complex thoughts. In musical theater, the writing that S.S. puts out is the best you’re going to find.”

Sondheim, for his part, participated in two panels, one featuring two of his frequent collaborators, writer John Weidman and writer-director James Lapine.

He talked with them about the changes musicals go through -- noting that “once you know what the shape of a show is, and the strengths and weaknesses of the performers, it’s much easier to write.” He recalled how the scene from “A Little Night Music” that includes “Send in the Clowns” changed form during tryouts in 1973, shifting focus from a male character to the female one who sings the now-classic song -- and how once it all became clear, the actual writing of the song took only a day and a half.

In the other panel he joined, “Sondheim and American Popular Culture,” the lively, diverse discussion included “Buffy the Vampire Slayer,” his love of movies, and the fact that “Desperate Housewives” names its episodes after his songs.

After the final number Saturday, Sondheim heartily blew out the candles on a cake that had been wheeled onstage and offered emotional thanks to everyone surrounding him, backstage and in the audience.

His actual birthday is Tuesday, and a gala evening, with expensive tickets, will be held on Broadway tonight. But as someone who had been in line Saturday for six hours remarked, “Sondheim is very much part of this city,” and “Wall to Wall Sondheim” was a distinctly openhearted New York way of saying happy birthday.