NEW YORK — When Iggy Pop's pants start to fall down, don't expect him to pull them back up.

Less than halfway through Iggy and the Stooges' first New York City performance since the early '70s, half of his butt was clearly visible to the audience. It was only through some miracle that his jeans remained above his privates for the rest of the show. But that's not something he thinks about.

The key to Pop's appeal is that he's a creature of instinct, not thought. He probably wasn't planning to leap from the Roseland Ballroom stage, climb into the VIP section and knock over tables during "Little Doll," but when Iggy is gripped by impulse, look out. 

Joined by the mechanical clamor of the Stooges, his Detroit band that pre-dated punk in the late '60s with loud, hypnotic riffs, garage rock bluster and attitude galore, Iggy's presentation was as unpredictable as it was captivating. No, he wasn't as wild as when he used to smear his body with peanut butter and cut himself with broken glass, but Iggy's grown up a little since then. He may no longer be truly unhinged, but he's certainly untamed.

He also remains a true nightmare for security. At various points in the show, he dived into the crowd with no warning, climbed atop monitors and dangled on a rope ladder meant for stagehands. He even invited members of the crowd to join him, and a flood of fans mobbed the stage.

Although Pop rocked with the ferocity of a cyclone, he exhibited little grace as he flung himself around the stage. But the raw magnetism he exuded as he danced, stomped, shook his torso and flailed his arms was awe-inspiring.

The Stooges backed Pop's revelry with chugging, repetitive rhythms that increased in power the longer they continued. Beefy guitarist Ron Asheton complemented the chaos with garage-y, angry and psychedelic solos, while his brother Scott drove the monochromatic beats on the drums. Keeping the pulse was new recruit Mike Watt (ex-Minutemen, Firehose) on bass. None of them moved much, allowing Iggy to hog the spotlight — at least when it was on.

During one song, Iggy asked that all the stage lights be shut off. In complete darkness, the band vamped and Pop shouted, "Blackout! Blackout! F--- you! F--- me. Satan. Schwarzenegger."

Much of the set was composed of favorites such as "No Fun," "TV Eye," "1969" and of course "I Wanna Be Your Dog," all delivered with freight-train intensity. The only song the Stooges played from Iggy's upcoming album, Skull Ring, was the title cut. (The disc, which includes four new Stooges tracks, comes out November 4.)

Godsmack opened the show with a brief acoustic set made up of tracks from the band's upcoming "unplugged" album, The Other Side, which was recorded last month in Hawaii. The set included two new songs as well as group standards such as "Serenity," "Awake" and "Keep Away." Stripped of guitar rage and percussive might, the tunes sounded almost pensive.

Frontman Sully Erna and his bandmates sat on stools during most of the tunes and looked as if they were aching to jump free and rock out. Even in acoustic form, Godsmack's music was filled with mystical energy and atmosphere; even without a wah-wah pedal, guitarist Tony Rombola let rip with some tasteful, bluesy solos. For two songs, Godsmack were joined by two members of Dropbox, a group Erna signed to his label. The additional musicians graced the songs with a little extra depth.

Whether Erna was just playing with Rombola or rocking with the full band, Godsmack sounded more like their mentors Alice in Chains than ever. That's just fine with Erna; the inspiration to record an acoustic album came from groups such as Led Zeppelin, Nirvana and most of all Alice, who released the acoustic classics Sap and Jar of Flies — two discs that made Erna who he is today.

For more sights and stories from concerts around the country, check out MTV News Tour Reports.