ABBA stars surprise Mamma Mia! audience

Benny Andersson and Bjorn Ulvaeus have attended Mamma Mia for the show's 20th anniversary.
Benny Andersson and Bjorn Ulvaeus have attended Mamma Mia for the show's 20th anniversary.

Abba's two leading men have made a surprise appearance as hit musical Mamma Mia! celebrated its 20th anniversary.

Benny Andersson and Bjorn Ulvaeus, one half of the Swedish pop sensation, delighted audience members on Saturday night as the musical marked two decades in London's West End.

The pair, who wrote the music and lyrics to the popular show, sparked huge applause and cheers from a packed Novello Theatre in London when they took to the stage after the performance.

Ulvaeus told the crowd it was "both strange and wonderful to be on this stage tonight" and thanked them for being "a fantastic audience".

They were joined by producer Judy Craymer, director Phyllida Lloyd and writer Catherine Johnson, who Ulvaeus described as "three angels".

Celebratory confetti rained down on the theatregoers during a medley of Abba classics including Dancing Queen and Waterloo.

Speaking backstage, Andersson said it had been "too long" since he had seen the show and revealed he felt moved on the anniversary.

Praising the cast as "wonderful", he added: "There is a chance to be immersed, to get moved and I was tonight and I'm happy that I can feel that."

The show, which has been seen by more than nine million people and played more than 8,000 performances, originally opened at the capital's Prince Edward Theatre on 6 April 1999, before transferring to the Prince of Wales Theatre in 2004, and to its current home in 2012.

Globally the musical has been seen in 50 productions in 16 different languages, and grossed more than $US4 billion ($A5.6 billion) at the box office.

Australian Associated Press