LONDON - About 200 music fans were treated to an impromptu set by U2 stars Bono and The Edge at a small London venue on Friday.

Watched by fellow band member Adam Clayton, the two Irish rockers delivered a 20-minute acoustic set at the Union Chapel in north London, in a gig for Mencap as part of the charity's Little Noise Sessions.

The duo played four songs "Stay", "Desire", "Angel of Harlem" and "Wave of Sorrow", the last song inspired by Bono's 1985 trip to Ethiopia.

"A really important time in the direction of my life and of the band," said the singer on U2's Web site.

BBC Radio One disc jockey Jo Whiley, whose younger sister Frances is disabled, jokingly introduced The Edge as a guitarist who had not been playing that long and might make some mistakes and Bono as a little bit shy.

Four more Little Noise Sessions are scheduled with Snow Patrol due to perform on Sunday and Keane set to appear in Wednesday's closing gig, as well as more surprise guest appearances promised over the coming days.

Mencap helps people with learning disabilities and their families.