Coachella Cameos: Who Took Crowds By Surprise

It is the artists who weren't expected at California's Coachella Festival who have garnered the most attention this year.

While enjoying scheduled appearances by acts such as Outkast and Muse, crowds at the huge site in Indio have also been treated to a number of surprise performances which have got them going wild.

Beyonce joined her sister Solange during her set for the song Losing You where the pair performed an energetic dance routine with beaming smiles on their faces.

Also not willing to just sit back and enjoy the festival atmosphere, Beyonce's husband Jay Z had earlier made a surprise appearance on stage with his former rival Nas.

Pharrell Williams enjoyed duets with a range of music stars including Puff Daddy, Busta Rhymes and Snoop Dogg but it was Gwen Stefani's guest appearance to sing Hollaback Girl which was most unexpected.

The singer is pregnant with her third child with husband Gavin Rossdale.

Fans whipped out their camera phones quickly when Justin Bieber suddenly appeared on stage with Chance the Rapper after teasing his fans with a Twitter post saying: "What am i up to today?? ;)"

And Arcade Fire closed the final day of the first weekend with a cameo from Blondie singer Debbie Harry, who performed her classic 1979 hit Heart of Glass with frontwoman Regine Chassagne, and the band's song Sprawl II (Mountains Beyond Mountains).

Coachella runs over two weekends, with the second half of the festival to take place from April 18-20.