Swift's 'unforgettable' UK show
The Blank Space singer, 25, could not hide her emotions as she declared her final UK show "unforgettable" when she performed at the British Summer Time Hyde Park festival in the capital.
Delevingne, who appears in the music video of Swift's song Bad Blood, waved a Union Jack flag as she joined the singer on stage for Style.
Tennis champion Serena Williams, who is competing at Wimbledon, and models Kendall Jenner - whose brother-in-law Kanye West is headlining Glastonbury - and Gigi Hadid also walked the stage after being introduced by the singer as "my friends".
The London festival marked the star's third and final date in the UK, following sold-out gigs in Glasgow and Manchester.
"I remember seven years ago when I played my first UK show," Swift told the 65,000-strong crowd.
"King's College had sold out and there were a few hundred people. I was surprised that a few hundred people would come to see me perform and now I'm looking at 65,000 people in Hyde Park.
"This is one of those nights where I'm aware I'm going to be taking mental snapshots so I can remember this night for the rest of my life. I love hearing you sing, dance and jump around. I will remember this night for the rest of my life."
Swift - who was named Forbes' most top paid couple alongside boyfriend Calvin Harris - opened the gig with Welcome To New York in a green sequinned jacket, crop top and purple skirt, before changing to an all-black ensemble.
She thrilled fans with her numerous costume changes during her energetic set, at one point wearing an LED-lit dress, a white crop top and shorts with stockings, a black catsuit and a sparkly dress as she sang tracks from her latest album 1989 as well as older hits such as I Knew You Were Trouble, Love Story and We Are Never Ever Getting Back Together.
"I love London in general. I love the sight of the London crowd in the summertime," she said during her track New Romantics.
"This is one of the best cities in the world."
As she closed the show with Shake It Off, she said: "This has been the best night. It's been unforgettable."
Before Swift took to the stage, Ellie Goulding, John Newman, Vance Joy and Rae Morris performed in Hyde Park.
The festival, which has seen headline performances from Blur, Kylie and The Who, ends today.