Justin Bieber Makes Surprise Appearance Onstage With Jack U

The dance superstars' headlining set, performed under the band name Jack U, featured a string of special guests, including singer Aluna George and rapper 2 Chainz, who hit the stage for renditions of their respective tracks To U and Febreze, but it was Bieber's addition to the line-up which really sent fans into a spin on the final night of the two-day festival.

The crowd at the Fairplex venue in Pomona erupted into screams as the 21 year old, wearing a Metallica T-shirt, stepped into the spotlight and launched into his Jack U song, Where Are U Now, leading the audience in a mass sing-a-long.

Bieber had hinted at the appearance earlier in the day, taking to Twitter to write, "It's a hard summer @Skrillex @Diplo."

After the show, his manager, Scooter Braun, tweeted, "Yep. @justinbieber came out for #hardfest and place went crazy. Love to @skrillex and @diplo for the invite."

Other performers at this year's (15) HARD Summer Festival included Years & Years, Die Antwoord, Fetty Wap, Rae Sremmurd, and fellow headliners The Weeknd and The Chemical Brothers.