Jon Hamm joins Zach Galifianakis for surprise set at Bonnaroo

Actor Jon Hamm showed off his singing skills as he joined comedian Zach Galifianakis at the Bonnaroo Music and Arts Festival for a surprise appearance on Saturday (13Jun15).

The former Mad Men star was a special guest at the Tennessee festival's annual SuperJam, a late night concert in which artists perform classic covers.

Saturday night's theme was 80s Throwback SuperJam, and Hamm and Galifianakis kicked off the event by leading the crowd with a rousing rendition of the 1985 USA For Africa charity single, We Are The World, which famously featured a star-studded roster of celebrities such as Michael Jackson, Ray Charles, Bruce Springsteen and Cyndi Lauper.

Earlier in the day, Hamm also surprised fans during Galifianakis' stand-up comedy set, where The Hangover star played a dog and Hamm portrayed his owner, feeding him special treats.