Bjorn Ulvaeus says ABBA success humbling as he marks two milestones

ABBA's Bjorn Ulvaeus reflected on the Swedish pop group's reach and longevity as he joined "Mamma Mia!" cast and creators for the musical's 25th anniversary celebrations in London.

It also marked 50 years since ABBA won the Eurovision Song Contest final in Brighton, United Kingdom, in 1974 with the song Waterloo, bringing them to global attention.

"About this time in the evening, exactly 50 years ago, I was standing on another stage in another city here in the UK," Ulvaeus said.

"It's strange to think that if we hadn't won... I most probably wouldn't be standing here today. And this wonderful adventure which we call Mamma Mia! wouldn't have happened," he said, speaking to the audience on the London stage.

ABBA was formed by Ulvaeus, Andersson, Agnetha Faltskog and Anni-Frid Lyngstad in Stockholm in 1972.

Mamma Mia!, composed by Ulvaeus and Andersson and based on their songs, originally opened in London's West End on April 6, 1999. Written by Catherine Johnson and directed by Phyllida Lloyd, it centres around a mother and daughter with three possible fathers.

According to its creators, over 70 million people have seen productions of the show in more than 450 cities around the world, staged in 16 different languages. It has also led to two blockbuster movies.

"The fact that somehow ABBA has managed to touch so many millions of lives around the world, generation after generation and people ask me 'how does it feel for you to know that?', and that's a very good question and very hard to answer," Ulvaeus, 78, said.

"It's a very elusive feeling. It's more to do with gratitude and with humility than pride, because it humbles you to know that so many people have listened to something you've created and that they've been made happy by it or sad, and that it has meant so much for them in their lives."

"It's very difficult to fully emotionally grasp that, at least for me," said Ulvaeus, who was joined on stage by producer Judy Craymer, who first met him and Andersson in the 1980s and convinced them that a musical could be made from their songs.

With its 25-year run, Mamma Mia! becomes the third longest running musical in West End history, after Les Miserables, which made its debut in 1985 and The Phantom of The Opera, launched a year later, in 1986.