Tough act to follow: U2's Bono, Edge warm up crowd

LONDON -- U2 stars Bono and The Edge showed up unannounced and played as a warmup act at a charity concert in London.

The show delighted the 250 people who attended the fundraiser Friday night for Mencap, a charity that campaigns for equal rights for children and adults with learning disabilities.

"Don't tell Larry (Mullen) and Adam (Clayton) we've done this," Bono joked about his other two bandmates from the small stage at London's Union Chapel.

Bono and The Edge -- performing as a warmup act for Biffy Clyro, a Scottish rock group -- played four of their songs: "Stay," "Desire," "Angel Of Harlem" and "Wave Of Sorrow," a track written for their 1987 album "Joshua Tree."

A member of the audience, Simon Dowling, 20, from Newcastle, northern England, said he was shocked when the U2 stars suddenly appeared on stage.

"I've used all the battery on my phone taking pictures of them," he said.