Film star Bradley Cooper made a surprise appearance on Glastonbury's Pyramid Stage, as he shot a scene for his new film.
The Hangover star is currently directing and starring in a remake of A Star Is Born, the Judy Garland-starring musical about an up-and-coming singer.
He appeared at Glastonbury ahead of Kris Kristofferson's set, holding a guitar and followed by cameras.
Cooper and his co-star Lady Gaga shot similar scenes at Coachella in April.
Before leaving the stage, Cooper thanked the audience: "You guys were awesome, that was great - it's my sheer pleasure to introduce Kris Kristofferson."
The festival opened on Friday morning with a minute's silence, to honour those affected by the recent tragedies in London and Manchester.
The moment of reflection was led by New Order's Peter Hook, whose daughter attended the Ariana Grande concert in Manchester last month, where 22 people died.
"Can we please use this as a chance to send our hopes and our prayers for love, life and freedom," he said. "[They're] the things we're all here to celebrate.
"We send our sympathies to everyone affected by the events in London and Manchester, and everyone affected in Grenfell tower. God bless you all."
As the silence ended, Happy Mondays singer Rowetta, appearing alongside Hook in a Manchester T-shirt announced: "You got the love".
They then joined Hacienda Classical - a collaboration between Manchester DJs Graeme Park and Mike Pickering and the Manchester Camerata orchestra - playing souped-up versions of dance classics.
The Pretenders opened The Other Stage, drawing a huge crowd to hear hits including Brass In Pocket, I'll Stand By You and Back on The Chain Gang.
Chrissie Hynde, wearing a Motorhead t-shirt, dedicated the song Hymn To Her to organisers Michael and Emily Eavis, and paid tribute to the late Clash frontman Joe Strummer.
"If he's still anywhere near this planet, he's here today," she said, as novelist John Niven watched from the side of the stage.
He had a recent Twitter spat with JK Rowling over the language he used to describe Prime Minister Theresa May.
In the afternoon, pop star Dua Lipa drew one of the biggest crowds so far to the John Peel stage.
Thousands of people packed out the tent, with the audience stretching beyond the perimeter, as she played hits including Hotter Than Hell, Be The One and Blow Your Mind (Mwah).
Speaking to the BBC as she came off stage, the 20-year-old was clearly overwhelmed.
"It's been my favourite, favourite performance on stage I've ever done," she beamed. "I've never seen so many people.
"It was just the most magical day in my life. Maybe even the best day of my life, apart from the day my brother and sister were born."
However, Dua threw herself into the performance so heavily she caused herself an injury.
"I've mashed my tailbone," she said. "It was already bruised, then I went on stage and danced like I'd never danced before. Now I can barely walk."
Later on Friday, Radiohead top the bill, exactly 20 years after their first headline appearance.
Guitarist Ed O'Brien said they would "leave their ego at the gate" and embrace the spirit of the festival.
Footballer David Beckham is also due to visit Worthy Farm on Friday, after organiser Michael Eavis invited him to open a local social housing project.
Although Friday marks the start of festivities, revellers have been arriving since gates opened on Wednesday.
Those in the know were treated to "secret" sets by indie band Circa Waves and alt-pop experimentalists Everything Everything on Thursday night at the small Williams' Green stage.
For Everything Everything, the short, 40-minute performance gave them the opportunity to revisit the scene of "the best gig we've ever had in our lives".
Two years ago, the band played the stage in the same week they released third album, Get To Heaven, "and in the time that elapsed, the listeners had learnt all the words and were singing them back", singer John Higgs told BBC News.
"It was a really amazing, moving moment for us all."
Thursday's show was almost a re-run of that show, as fans joined in with the band's new single, Can't Do.
Although their set was not part of the official line-up, several thousand fans got wind of the show, and the audience spilled out of the tent into the surrounding fields.
"I keep getting texts from people I haven't seen for years saying, 'Oh, you're playing tonight, aren't you?'" joked drummer Michael Spearman.
"You can't really keep anything secret, not in this day and age," added Higgs.