Conan O'Brien and Steve Martin play rock stars at Bonnaroo

Comedians were living out their rock star (or bluegrass star) dreams throughout the first full day of the Bonnaroo Music & Arts Festival.

And they showed they can carry a tune, too.

Conan O'Brien started the day with a 1 p.m. performance in which he often traded a microphone for a guitar. Later, Jack Black's Tenacious D took Bonnaroo's enormous main stage, a fittingly grand backdrop for their mock, over-the-top theatrics. And Steve Martin performed with his bluegrass band, the Steep Canyon Rangers, for a set where bluegrass was the main attraction, and one-liners merely between-song banter.

While all of them used comedy in their music to varying degrees, they were also sincere about rocking out.

O'Brien, the former "Tonight Show," host, brought his "Legally Prohibited from Being Funny on Television" tour to the Tennessee music festival. He played to a packed comedy tent that, before he hit the stage, chanted "CoCo" — the nickname that he earned during his "Tonight Show" run and became the rallying cry for his fans when NBC ousted him.

As O'Brien has done throughout his cross-country tour, he frequently rocked out, performing songs by Elvis, the Band, the White Stripes, Willie Nelson and others.

The lanky, red-haired O'Brien fit in well at the music festival. If one wandered into O'Brien's act during his earnest encore of the Band's "The Weight" — when he leapt from the stage and walked through the crowd — his show may have appeared to be just another music act.

But there were plenty of jokes, too.

"In six months, I've gone from hosting the 'Tonight Show,' to performing at a refugee camp," O'Brien announced at the top of the show.

Bonnaroo draws 75,000-plus fans to a giant farm south of Nashville. Heavy rain the day before the festival opened Thursday night muddied the grounds, though excessive heat dominated Friday's performances. This year's Bonnaroo, the ninth annual, is on track to be one of the hottest. The heat index — a combination of temperature and humidity — is forecast to be near 100 for much of the weekend.

O'Brien made light of the surroundings and of the experience playing his first music festival — "unless you count the time I went stage diving at the Lilith Fair," he joked.

"I love doing comedy in a tent, at one in the afternoon, for people who haven't slept in two days," O'Brien said wryly.

O'Brien is a talented guitarist (he has often worked guitar-playing into his late-night shows), and he flashed his skills with a solo on the White Stripes' "Seven Nation Army." He was backed by most of his "Tonight Show" band.

The White Stripes have a long, friendly history with O'Brien. After years of appearing on his shows, it was O'Brien's turn to play on the band's turf. On Thursday night, O'Brien performed a rockabilly set at Nashville's Third Man Records, the label owned by Jack White. The show was recorded and will be released on vinyl.

For years Bonnaroo has included somewhat of a mini comedy festival, with about a dozen comedy acts alongside its 100 music acts. When Chris Rock came to Bonnaroo in 2008, he played the much larger main stage.

Ashley Capps, co-founder of Bonnaroo and president of festival co-producer AC Entertainment, said organizers discussed putting O'Brien's show on another stage but decided the comedy tent made the most sense, partly because of its video segments. The show was broadcast on screens in two other tents, as well.

O'Brien, who begins a new show for TBS in November, will perform at Bonnaroo again Saturday, and is playing master of ceremonies on the main stage Friday and Saturday, as well.

Martin made the banjo a central part of his standup act decades ago, but bluegrass has in recent years become a more serious endeavor to him. Last year, he released "The Crow: New Songs for the 5-String Banjo," which won a Grammy earlier this year for best bluegrass album.

"It's been a longtime goal of mine to play bluegrass at Bonnaroo," Martin deadpanned to the eager crowd. "Tonight, I feel like I am one step closer to that goal."

The young audience, most of whom weren't particularly versed in bluegrass, waited patiently through the songs, enjoying the lilting, old-timey sounds of Martin and the Steep Canyon Rangers. But they guffawed at Martin's dry quips in between.

"You realize I have not Googled myself for over 30 minutes," said Martin, shortly after consulting his set list on an iPad.

Of course, there were plain old musicians at Bonnaroo on Friday, too.

The southern rock band Kings of Leon headlined the main stage Friday night, an honor that will be followed by Jay-Z on Saturday and Dave Matthews Band, to close the festival, on Sunday.

Brooklyn's the National, a band not necessarily known for raucousness, worked up a frenzy, performing an expansive set that included horns and strings. While the Flaming Lips' Wayne Coyne excitedly looked on, the band slowly built their songs until a final rush of catharsis.

Singer Matt Berninger busted a mic stand, frequently jumped down to the crowd (he thanked "men with strong arms and women with strong grips" for helping him back up) and occasionally shouted to the crowd without a microphone, no amplification needed.

At Bonnaroo, where sets extend deep into the night, there were more performances to come for those who weren't done in by the heat. Among the several acts slated for after midnight were the Black Keys, LCD Soundsystem and the Flaming Lips, who were to play Pink Floyd's "Dark Side of the Moon."