CHICAGO — Every festival needs a sure-fire headliner that will bring in the huge crowds, if only for just one night. This year's slam-dunk was Saturday night capper Mumford & Sons, and the British folk revivalists were more than up to the task, drawing a massive, foot-stomping throng to the South end of Grant Park for a string-plucking run through their catalog.

They helped shut the lid on a spectacular cool and sunny day by Lake Michigan that featured everyone from Ellie Goulding to Mr. "Harlem Shake" himself, Baauer, as well as the National, the Lumineers and the Postal Service.

From the look of the swarm of humanity that stretched several hundred yards form the stage, though, Mumford were the act to beat. With smoke billowing across the stage, they wasted no time, busting out breakthrough hit "Little Lion Man" right away to grab their fans and get them moving.

On the same stage that has previously hosted plugged-in thrash by everyone from Pearl Jam to the Red Hot Chili Peppers and Rage Against the Machine, the Mumfords did what might have seemed improbable, if not impossible: they kept the audience's attention with their patented low-key, busk-y mix of banjo, mandolin, acoustic guitar, stand-up bass and drums. If anything, their biggest challenge was to simply rise above the constant, not-so-dull roar of incessant chatter from their admirers.

Seeming fully recovered from recent brain surgery that forced the cancellation of earlier festival dates, bassist Ted Dwane sounded a steady thrum on "Holland Road," looking healthy and energetic during the entire set. A short time later, after starting off soft, "Below My Feet" rose to a roar, a pattern that was repeated often during the nearly two-hour performance.

It's hard to pull off four-part a cappella harmonies and finger-picked acoustic guitar ballads for 40,000-plus people, but somehow M&S pulled it off on songs like the plaintive "Timshel." Casual fans were rewarded mid-way through with the band's usual set-closer, raucous hit "I Will Wait," a fast strummer that brought uplifted cell screens to life as dozens of girlfriends were hoisted up on shoulders to dance and bounce amid a sea of quick-clapping hands.

The traditional nightly fireworks display exploded above the stage, lighting up the night as singer Marcus Mumford took a seat behind the drums for "Lover of the Light" and the band served up what might be the first-ever straight-up banjo solo by a major headliner at Lolla.

Every time it felt like the lads might be losing the room, they cranked out a rollicking jig like "Roll Away Your Stone" and the crowd got to bouncing and clapping again. And though it looked like close to half the field had already emptied before the set came to a close, Mumford tested those who stayed with what he promised was a "very, very, very quiet" song that would require them to "shut the f--- up" for a few minutes.

He wasn't kidding, as the band gathered around a single mic for a campfire jam-like cover of the Bruce Springsteen ballad "I'm On Fire." Their patience was then rewarded with a spirited run through early hit "The Cave," which sent the faithful bounding off into the teeming streets of downtown.

That set capped a day that featured an early treat in the form of 64-year-old soul slanger Charles Bradley, whose Otis-Redding-meets-James-Brown pleading and shouting was as electrifying as his indigo-colored jumpsuit with a bedazzled gold Pharaoh's head on the back. And not for nothing, but while rocking the jam "You Put a Flame On It," he totally did the robot into a split, so that ruled.

As the first few notes "The Art of Peer Pressure" rang out, hordes of people ran toward the North stage for an especially hyped-up set from Kendrick Lamar. The Compton MC got the main stage crowd bouncing and shouting to "P&P," his verse on A$AP Rocky's "F---in' Problems" and his smash, "Bitch Don't Kill My Vibe."

Sister act Haim got bumped up to a more prime time dinner slot thanks to the no-show by Death Grips and they made the most of it by showing off their unique sibling chemistry and versatility. From the staccato strutter, "Better Off," to a seamless pass-the-mic style that gives each one a chance to shine a bit, the three siblings served up everything from Chicago-style blues, to the funky rock of "Falling," during which lead singer/guitarist Danielle ripping off some Prince-like solos.

Lollapalooza is going on all weekend with sets from everyone from 2 Chainz, Icona Pop, Phoenix, Kendrick Lamar, Mumford & Sons, Lana Del Rey and many others. Stick with us for news, pictures and interviews from the festival!