High on the wall, a gold-tiled mosaic sparkles in the glow of chandeliers suspended from a soaring ceiling. But it can’t compete with the living mosaic below. Posed on tiered scaffolding, elbow to elbow, are 120 people of all colors, shapes and ages, most of them naked.

It is a fleshly fantasy framed in marble columns, an encyclopedia of humanity — or at least a cross section of northwest Brooklyn.

“Stefano, if you can sit down on the platform with your legs dangling forward,” calls out Sarah Small, 32, the Washington-born photographer-ringleader who hatched this massive performance-art project. She calls it a “tableau vivant,” or living picture, the kind of thing that was popular in the Victorian era as a way to re-create famous visual works, often with an erotic undertone.

Small’s work is both a throwback to that and an experiment in pushing the modern limits of privacy. On this chilly, overcast Sunday a few weeks ago, with gray light filtering through the stained-glass windows of the former Williamsburgh Savings Bank, Small is rehearsing her art models in what will be a meld of music, movement . . . and two weddings.

In response to her request, a bald, burly, naked man hops down a few steps to the scaffold’s central level, then squats and scoots to the edge with his legs wide apart, hands on knees.

Small considers him solemnly. “Yeah, I like that,” she says. Long-limbed and slender, with a pale oval face and a mane of brown hair, she could pass for a dancer in her black tank top, leggings and boots. A tattoo encircles her right biceps; her fingers glint with silver rings.

She tinkers with a few more of the poses, composing mini-dramas among the models that convey conflict, or isolation, or comfort. Most of the models are in various states of recline. A few crouch, grimacing, with hands clenched like paws. Two young blondes nestle together, one topless, the other not. At the center of the display is a pair of spectacular nudes: a heavily obese woman sprawling next to a thin man curled head to knees. All we see of him is his folded posterior, mooning us, and vertebrae popping up like a string of beads along his back.

Small, a graduate of the Rhode Island School of Design, has been gaining recognition for her camerawork — her photos have appeared in Vogue, Life and Rolling Stone, and in galleries throughout the country as well as in Europe and Asia. In 2009 she started composing living installations to promote the ongoing photographic series she calls “Delirium Constructions,” tightly focused portraits that capture high emotion the moment it bursts out of her subjects.

Everything about this current project is huge: the cathedral-like art deco space; the number of models involved, all volunteers; the sponsors, including Michael Huffington, Arianna’s ex. Even the title is a mouthful: “Tableau Vivant of the Delirium Constructions: A Live Exploration of Implausible Interaction.”

Small also heads a Balkan a cappella vocal quartet, called Black Sea Hotel, and it will perform on the scaffold with the models, along with a string quartet. She has brought in a music director, a stage director and a choreographer. A film crew is following her around for a planned documentary of her work.

But what is most interesting is that for all its grandeur, this project is not an ego trip. Most performance art relies heavily on its creator’s personal magnetism — think of that veteran of the field Marina Abramovic, famed for her marathon appearances that encourage close contact with audiences, or recent works such as “Naked” by the Japanese American duo Eiko and Koma, in which they put themselves on display for weeks on end.

Yet Small barely figures in her tableau. She will appear among the models at certain points during the hour-long performance, conducting their movements as if they were a giant vertical orchestra. But the audience will barely see her; she will disappear among her masses, and that’s the point. Small’s tableau turns art-world egomania and our present-day fixation with ourselves on its head. She has created a major opus that is surprisingly self-effacing.

Here, mankind itself is the star.

“This piece is built by the people who are in it,” Small says. “I never come with pre-set ideas.”

What interests her is “the human quest for intimacy.”

“It’s exciting to be able to promote intimacy,” Small continues. “That’s probably why I make art, to open up a fleeting moment in time to be able to share something intimate. On the one hand it’s like manufactured intimacy, and using that kind of language it sounds like it’s fake, but it’s so not.”

This is why she doesn’t consider her tableau a form of site-specific theater.

“I’m not making theater. I reject the idea of theater,” she says. “I feel like in theater there’s not much room for on-the-fly personal expression.”

She got the idea of including nuptials in her tableau when she was told that at her first choice of a venue, the Brooklyn Museum, a curatorial committee would take years to review her request. But if she were doing a wedding, the space could be immediately available . . .

“And I was like, genius!”

“As this piece is so much about the human experience,” Small says, “and marriage is about that, too, it seemed like a really good fit.”

She shelled out $59.99 to get her online certification as an “esoteric spiritual minister,” along with a New York license, so she can legally officiate at the weddings of a pair of models on each of the two nights of the event. They’ll take their vows on the scaffold after Small clambers up to join them.

She didn’t get the Brooklyn Museum, but it’s hard to imagine a more impressive framework than this bank building, with all its hard, gleaming symbols of wealth and commerce. Against its marble walls, Small’s models look at once proud, vulnerable — and a little hippie-hooey. Think of the sprawling humanity in “The Garden of Earthly Delights,” by Hieronymous Bosch, or Gustav Klimt’s entwined lovers, one body indiscernible from another. Small’s painting incarnate feels a little San Francisco, with more than a whiff of the ’60s — but for the prevalence of 21st-century tattoos and nipple rings.

“Nudes, please put your robe on if you’re cold,” Small announces to the group. “And then resume your poses. With expression, full on: Give us everything.”

In a single wave of action, they comply. The models move into a slow game of freeze-tag. A young black man in trousers, white shirt and tie drapes an arm around the shoulders of an older naked white man. Dotted throughout the assemblage are the musicians, who will play later on. Among them is a bearded cellist, stripped down to his socks, Bulgarian singers in colorful folk dress and, perched on the highest level, two sopranos — one corseted into an 18th-century-style robe a la Francaise, the other wearing nothing but blue eyeshadow and a hair ornament.

The vintage check-writing tables in the center of the bank’s grand hall have marble pedestals and are topped with thick green glass. A couple of pantsless participants in the tableau are in seated poses on the table nearest the risers.

You hope the evening’s cleaning crew will be using something stronger than Windex.

“Who in their early 30s puts on something on this scale?” says Abigail Wright, a mezzo-soprano and 2007 University of Maryland graduate who has sung in several of Small’s installations. (She’s the naked one in this piece.)

“I mean, who does this? Getting 120 people to bend over backward for you, clothed or unclothed — that takes commitment. This is hard, this is vulnerable, this is scary.

“The stakes keep getting higher,” Wright says. “And Sarah just gets better every minute.”

But then, ambition is hardly new to Small.

“Ever since I’ve known her,” says Small’s father, Haskell Small, “from the moment she popped out of the womb, she has been the person to say, ‘Do this’ or ‘Do that.’ ”

Small grew up in Wesley Heights, near American University. Her parents are both musicians. Her mother plays the lute; her father is a pianist and composer. His painting-inspired composition “The Rothko Room: Journeys in Silence” premiered this February at the Phillips Collection.

At 13, Small discovered photography at summer camp. She had a crush on a boy, and in the darkroom she stumbled upon not only a calling but also her first kiss. Back home, she got hold of a basic Pentax camera and turned the lens on her younger sister, Rachel. At the Field School, she talked her way into taking sports photography instead of gym.

“I’m somebody who loves to closely study human interaction and different kinds of emotional expression,” Small says. “I read people all the time.”

After moving to Brooklyn in 2001, she developed an eye for visual contrasts, subway microcosms, the way rich and poor might share a bench but never interact.

Small says she has been more influenced by music than by other visual artists; she took up Balkan folk singing because of the thick sonic layers and dissonant chords, “and the moving in and out between resolve and tension, exactly what I’m trying to do in visual work.”

But she is a fan of performance artist Abramovic; Small waited in line for 50 hours to participate in “The Artist Is Present” at the Abramovic retrospective last year at the Museum of Modern Art. In that performance, Abramovic sat silently at a table for three months while museum visitors took turns sitting opposite her. What moved Small most was “this idea of reveal,” how looking at another person forced her to reflect on herself.

On the tableau’s second night, CJ Follini, 44, and Renee Ryan, 40, will renew their vows before Small and everyone else in the space. Follini chairs the board of directors of the HERE Arts Center, a space for experimental performance art in SoHo. Small’s persuasive powers — her ability to sweep people into her orbit — got his attention.

“I know what it takes to put on a show,” Follini says. “But to do that with 30, 40 models — and now 120, all doing something that conveys emotion — is amazing.”

It’s a few minutes before performance time on the first night, and an art-school-chic crowd fills the vestibule outside the bank lobby.

Inside, it feels churchlike. Quiet and candlelit, with the electric lights tinted purple. The risers have been swathed in white fabric, and the tiers look like a giant wedding cake. The models are curled up like napping children.

Singer Shara Worden, the one in the 18th-century dress, begins an aria. The models stir, arrange themselves into their poses. The Balkan singers start up, their costumes and lipstick a bright focal point. As for the models, you’re struck by how soft they look against the white drapery, how the skin tones resemble pebbles in a stream, earthy shades of buff, cocoa, ebony and oak.

Small emerges from behind one of the check-writing tables in a colorless chiffon dress with a wide sash. She climbs up the structure, reaching her arms out, and as she does so, the models begin to breathe audibly and undulate. Small makes her way to the bridal couple — Alexandrea Thomsen and Siddhartha Dillon. The bride is in a long white dress, the groom in a tux jacket and cargo shorts. Small huddles with them; you see their lips moving, they kiss, and all the models salute them with raised arms. You feel like applauding, but the performance goes on.

Improbably, the tableau works on a couple of levels. There is a strong visual dynamic — your eye wanders from one drama to the next, and the subtle textures of fabric and flesh, the skin taut or loose, are like daubs and ripples of paint. But what is most moving is the absence of judgment. Everybody — every body — is given equal attention. As Small in her little-girl dress moves among the models, she looks like a child in a dream-fantasy, arranging the adults as she pleases, creating her own new world.

Indeed, there is an energizing newness to this “Delirium Construction,” a humble and humbling view of humanity that feels authentic. Even poetic.

Somewhat miraculously, given the number of people involved and all the crescendoing music and chanting, what you take away from the tableau is a kind of peace.

Afterward, there’s an open bar and more Balkan music.

What’s next for Small? She wants to create a children’s tableau. (“Obviously,” she says with a grimace, “not with nudity, ’cause we live in America.”) There’s the documentary, due out in a year or so.

Right now, though, there’s that post-creative buzz. “I feel like I’m floating,” Small says. “I feel like I’m the one who got married! I feel high.”

Around her, the audience and her models have melted together, drinking, dancing, dressed or not. Some are in nothing but socks; Wright, the soprano, is wearing only sandals and chatting amiably with her fans. Thomsen sweeps by in her floor-length taffeta. You watch this crazy, wonderful living picture and think:

It’s delirium, all right — unconstructed.