Who’s the real ‘idol’?

How can hip-hop ever be the same after this? In its new album and in a ferocious, sexy performance on Thursday at the Wiltern LG, N.E.R.D. served notice this week that its effect on the broader pop culture could transcend its members’ already massive influence in the hip-hop and R&B; worlds.

Leaders Pharrell Williams and Chad Hugo (the third member is the MC known as Shae) are better known as mega-producers the Neptunes, winners of this year’s producer of the year Grammy for their work with Justin Timberlake, Jay-Z and half a dozen others artists who have dominated the urban charts.

But N.E.R.D., their recording and performing side project, is a full-blown arena rock band.

In their two albums, including “Fly or Die,” which came out Tuesday, they only hint at the rock energy that propels their live show. With Williams and Shae out front and Hugo behind keyboards Thursday, they conjured a power-soul sound that was earnest and aggressive.

When their 2001 debut album “In Search of ...” first came out, it was dismissed as an indulgent side-project by hip-hop purists and some leading urban DJs who were playing their other productions. But then N.E.R.D. (it stands for “no one ever really dies”) found a new crossover audience within modern rock. This was music made for KROQ-FM, but it gradually filtered into the hip-hop community, if largely on the force of Williams’ presence as a tastemaker, style setter and industry leader. A few songs, such as “Lapdance” and “Run to the Sun,” can now be heard in hip-hop clubs.

The arrival of “Fly or Die,” however, makes it plain: N.E.R.D. is a heavyweight in the hip-hop world, and heavy rock is now going to be part of it. N.E.R.D. works in a classic soul lineage that, like Little Richard or Sam & Dave or Parliament, embraces rock.

As hip-hop producers, they would be expected to lift catchy soul melodies and lay them over beats, but this is an entirely different process. With Williams’ falsetto, a fair number of the songs come out sounding like a progressive-rock take on “Superfly”-era Curtis Mayfield: guitar-driven, super-funky but also hard-bitten and serious.

Played live Thursday with the help of the Minneapolis piano-pop band Spymob, the title track of the new album was a new-wave analogue to Mayfield’s “Freddie’s Dead.” It’s a tale of teen rebellion buoyed by fast-moving distorted guitar, on which Williams rages: “It won’t be long ‘til you see me on the news / Fly or die, sink or swim, which one should I choose?”

“She Wants to Move” was a standout example of the new vibe. Williams worked a song of frustrated passion like Otis Redding, down on his knees to a woman he pulled out of the audience, squeezing every ounce out of it, then leaping up at the rock chorus and shouting to the surging crowd, “You know how good I feel right now?”

N.E.R.D.’s expansive vision even dwarfed someone who knows a thing or two about arenas, Justin Timberlake. He came out to wild crowd response to join opening act Black Eyed Peas on their joint hit “Where Is the Love,” then reappeared for about half of the set with N.E.R.D.

The Peas’ number is good, crowd-pleasing fare, a rap with a sweet, sing-along R&B; hook. But even the first piece Timberlake did with N.E.R.D., “Run to the Sun” rolled into “Stay Together,” showed N.E.R.D. working with an entirely different level of ambition.

With Williams pushing the songs physically from Prince-like soul to heavy, near-prog-rock choruses, it seems that N.E.R.D. has the potential to be accepted by both the purists and the masses. Unlike other rap experiments in rock crossover such as Ice-T’s metal band Body Count, N.E.R.D. is elusive, defying genres, and always one breakdown section away from hip-hop roots.

Late in the set, Timberlake beat-boxed through a tale of teen homelessness, “Bobby James,” and they teamed up for an emotionally pitch-perfect version of the 2003 Pharrell Williams R&B; solo hit, “Frontin’.”

In the end, though, it was pure pandemonium. Coming back out for encore versions of “Rock Star Poser” and “Lapdance,” Williams stripped off his shirt, and ecstatic fans flooded the stage, pogoing to a crunching guitar.

It was a hip-hop party. It was rock transcendence. It was sincere, visionary, and seemed to fall in a gray zone full of raw and untapped emotion -- all the hallmarks of a future classic.