About 250 music lovers were treated to a surprise performance by U2's Bono and The Edge when the Irish rockers showed up at a charity concert Friday in London, England.

The pair played a four-song set at a concert in the Union Chapel in north London to benefit Mencap, which helps people with learning disabilities and their families.

U2's Bono, right, and The Edge, left, seen here performing with Mary J. Blige in 2006, surprised a London audience Friday night with a four-song performance at a charity event.
(Mark J. Terrill/Associated Press)

"Don't tell Larry [Mullen] and Adam [Clayton] we've done this," joked singer Bono about his two missing bandmates.

The crowd were told about some "very special guests" but had no idea who it would be until the two walked out on stage. The multimillion-dollar musicians were the opening act for Biffy Clyro, a Scottish rock group.

"The singer, Paul, is a shy guy, so please be gentle with him," BBC Radio DJ Jo Whiley told the crowd, before Bono and The Edge — real names Paul Hewson and Dave Evans — appeared.

The two then launched into Stay and followed with Desire, Angel of Harlem and Wave of Sorrow, a song inspired by Bono's 1985 trip to Ethiopia. Wave of Sorrow was originally written for the band's 1987 Joshua Tree album but never made it.

It was over in 20 minutes and the pair left during their standing ovation.

"After they came off stage, Bono was asking me if they'd been OK and they were also wondering whether the 'new' song had gone down well," Whiley told BBC News.

"I've used all the battery on my phone taking pictures of them," said fan Simon Dowling, 20.

Biffy Clyro frontman Simon Neil said he was only told Friday morning that Bono and The Edge were to perform.

"We got to meet them earlier and they were very kind. They actually apologized to us for jumping on our show which obviously, you know, is incredibly polite."