It's hard to say which is more striking about Green Day circa 2005 -- the group's recent maturation or its sudden arena-rock tendencies.

Both were on full display Tuesday at Merriweather Post Pavilion, where Green Day performed material from last year's "American Idiot," a sharp, sociopolitical punk-rock opera that signaled a serious artistic growth spurt for a group previously known for caca jokes and songs about boredom, bong hits and masturbation.

The long-running Northern California band then punctuated the proceedings with a bunch of pyrotechnics: There were flash pots and torch flames and spark showers and such (and if there isn't a rule about turning in your punk badge once you reach double-digit pyro cues in a single show, well, there should be).

If the explosives weren't rawk-goddish enough for the kids in the crowd, well, thank goodness for the elaborate lighting and the disco ball and the wave-your-arms exhortations and the big blinking multicolored bulbs that spelled out GREEN DAY. There was also a cover of Queen's arena-rock anthem "We Are the Champions," an encore that was surprisingly light on irony. Somewhere, six feet under, Joey Ramone was rolling over.

So, Green Day: Definitely no longer a punk band -- although that story line has been in development at least since 1994, when the "sellout" charges came fast and furious upon the release of "Dookie," an MTV-friendly album that racked up sales of about 10 million copies.

There's also this, though: Green Day, one of the more compelling big-name rock bands around, by virtue of "American Idiot" (roughly 4 million sold -- less than half the "Dookie" haul, but a very big number nonetheless in the iPod era), along with a live show that sits at the entertaining intersection of good music and good theater.

Lest you worry that this means Green Day's, like, totally grown up and stuff, there was plenty of juvenile evidence Tuesday to suggest otherwise.

The manic lead singer and guitarist Billie Joe Armstrong half-mooned the crowd, pretended to pass gas with the help of the tenor saxophonist (yes, the band occasionally used a horn section) and blasted fans with a water gun. Armstrong also pretended to pleasure himself at one point -- though The Washington Post can't verify the "pretended" part. Besides the shenanigans, of course, there was also music.

Playing for the first time since Sunday night's MTV Video Music Awards, where Green Day collected seven statues -- including best rock video and video of the year, for its "Boulevard of Broken Dreams" clip -- Armstrong, bassist Mike Dirnt, drummer Tre Cool and several sidemen bashed, burned and sometimes wobbled through greatest hits ("Long View," "She," "Basket Case") and covers (Queen, Operation Ivy, "Shout," a bit of "Stand by Me"). But the majority of the set list featured songs from "American Idiot," including the album's first three tracks performed in sequence to open the nearly two-hour show.

The capacity crowd of 17,500 provided backing vocals, at very high volume, to just about every song out of the Green Day catalogue, including the album tracks that never made it as singles. And while the singalong occasionally annoyed, there were also moments of brilliance. Witness, for example, the slight young boy, maybe 8 or 9 years old, emphatically -- no, belligerently -- shouting "Can I get another Amen! There's a flag wrapped around a score of men!" from the antiwar screed "Holiday" as his mother stood stiffly and silently next to him, her arms folded, her lips pursed, her eyes rolling.

Now that is a punk-rock moment worthy of a flash pot.

Green Day's Billie Joe Armstrong still had a few sophomoric tricks up his sleeve at Merriweather Post.Green Day fans provided backing vocals for nearly every song the band performed at Merriweather Post Pavilion.