Photo: Weiner/Retna

When it comes to the Roots — currently in-residence as the house band for Late Night With Jimmy Fallon and arguably the greatest live hip-hop act on the planet — you must always expect the unexpected. So it should come as no surprise that Saturday’s second annual Roots Picnic at the Festival Pier in Philadelphia was a 12-hour all-you-can-eat-and-then-some feast of wonderful WTF-ness.

What other band would kick off their own multi-genre Lollapalooza — featuring Public Enemy, TV On The Radio, the Black Keys, Santigold, Kid Cudi and Asher Roth — with an impromptu improv with guest MCs Jordan Knight and Donnie Wahlberg of New Kids On The Block (who just happened to be performing later that night across the river at the Susquehanna Center in Camden)? Who else would dare back up Chuck D and Flava Flav for a beginning-to-end live rendition of Public Enemy’s 1988 landmark album It Takes A Nation Of Millions To Hold Us Back? Who else would book the Black Keys’ sludgy biker-blues choogle in between Ohio player Kid Cudi’s uber-catchy indie-rap and Santigold’s retro-pop readymades from the Ray Ban-ed dawn of MTV? Small wonder we saw so many ?UEST FOR PREZ T-shirts in the crowd.

After a typically jaw-dropping mid-afternoon set by the Roots made it official that the Picnic had begun, DJs Jazzy Jeff and Cash Money kept the main stage crowd entertained between sets with daring displays of turntabilism and old school jams. Next up on the main stage was Elevator Fight, featuring Hot Rock Offspring Zoe Kravitz (daughter of Lenny and Lisa Bonet) and Brooklyn Afrobeat all-stars Antibalas. Meanwhile, Philly-based rap pack Writtenhouse Square and weird-beard motormouth MC Bus Driver threw down in the crisply air-conditioned chill-out tent. Back on the main stage, Philly native Santigold — decked out in a purple, paint-spattered ’80s-rific jumpsuit and flanked by two gold lame-clad dancers in blue-and-white checkered wayfarers — rocked the body and soul with her patented M.I.A.-meets-Missing Persons joint. Next up was Akron, Ohio blooze twosome Black Keys who laid rubber for nearly an hour with an impressive set of whiskey-soaked shit-kicker rock.

Back in the chill-out tent, Kid Cudi led a couple thousand kids through a sing-along of his hit “Day And Night,” followed by Asher Roth’s odes to being white, suburban and in love with weed and hip-hop. The Roots and the Antibalas horn section brought the noise for Chuck D and Flava Flav — who was uncharacteristically hatless and shade-free, but rocking his trademark big clock medallion — as they set the Wayback Machine to 1988, when seemingly every passing car blared It Takes A Nation Of Millions To Hold Us Back, wrapping it all up with the one-two punch of “Don’t Believe The Hype” and “Bring the Noise” and encoring with a fist-pumping, head-crushing “Fight the Power.”

Artsy Brooklyn alt-rockers TV On The Radio, who have clearly learned a thing or two from PE’s mesmerizing wall of sound, were still in awe of the Nation Of Millions revival that preceded their set. In between prismatic renditions of “Shout Me Out” and “Dancing Choose,” TVOTR drummer Jaleel Bunton exclaimed, “Public Enemy — holy shit!” TVOTR frontman Tunde Adebimpe seconded the emotion. “Yeah, that’s how we all feel,” he said, and he might as well as been speaking for all 6,500 in attendance.

Apparently operating under the credo It’s Our Party And We’ll Smoke Everyone That Came Before Us If We Want To, the Roots closed out the festivities with dizzying, awe-inspiring hour-long jam. By our count, they only played three proper Roots songs — “You Got Me,” “The Next Movement” and “The Seed (2.0)” — but those were just jumping off points for a magical history tour across the continuum of popular music. Dub reggae, classic soul, soft-rock, hard-rock, garage-rock and cock-rock were all essayed in the stunning genre-morphing medleys (Bo Diddley’s “Who Do You Love,” George Thorogood’s “Bad To The Bone” Muddy Waters’ “I’m A Man” and Guns N’ Roses “Sweet Child Of Mine”) that the Roots inserted in between the verses and choruses of their own songs. If you weren’t there, know this: The Roots could be your iPod, if only you’d let them.