Watch Taylor Swift Join Haim On “Gasoline”/”Love Story” Mashup In London

Taylor Swift surprised fans by joining HAIM onstage at London's O2 Arena, where the group performed their collaborative track "Gasoline" with a mash-up of Swift's "Love Story." Once onstage, Swift said: "I haven’t been on stage in a very long time... It’s very nice [to be back]. When I heard my girls were playing in London at the O2, I thought, I’m going to have to see that. And it looks like there’s about 20,000 other people who also thought that."

Swift added: "So we had a thought. If we were to do some sort of mash-up, we could possibly maybe get you to sing the loudest you have sung all night -- which is a big challenge because you have been singing very loudly – extraordinary work."

Watch some fan-shot footage of the moment below.