The singer spoke out on Friday as the Supreme Court ended the country’s constitutional right to abortion.
The 20-year-old singer’s comments came as the US court decided to overturn the landmark 1973 Roe v Wade case that legalised abortion nationwide.
During her set on the Pyramid Stage on Friday night, she criticised the ruling as she introduced her song Your Power, which explores people abusing their power.
Seated alongside her brother and co-writer, Finneas O’Connell, she said: “The song we’re about to do is, I think, one of the favourites that we’ve written and it’s about the concept of power and how we need to always remember how not to abuse it.
“And today is a really, really dark day for women in the US. I’m just going to say that as I cannot bear to think about it any longer in this moment.”
This appearance was particularly poignant for the singer, as she became Glastonbury Festival’s youngest ever solo headliner.
During the electrifying set, she treated the audience to a selection of hit songs from her two number-one albums, including Bury A Friend, When The Party’s Over, NDA and My Strange Addiction.
She gave a typically energetic performance, running and dancing across the stage, wearing a black and white graphic outfit with black striped arm-warmers and her hair in space buns.
The singer also used her set to speak about how society needed to better protect the environment.
Ahead of performing Everything I Wanted, she said: “So I think we all know that we really need to do a better job of protecting the planet that we all live on.
“We always can do a better job of protecting our people and our friends and family and strangers and standing up for each other and standing with each other.
“And I love you all and I stand with you and I support you and everything that makes you who you are.”
The audience appeared to reciprocate back her affection as throughout the set they sang, danced and chanted her name.
Near the end of her set she admitted she felt “so undeserving” of this opportunity as she thanked the festival and the fans for having her.
She added: “I am so honoured to be here among all of you and all the incredible artists and the history of this festival.”
Eilish closed her history-making show with an explosive rendition of her hit track Happier Than Ever.
The singer energised the crowd as she delivered the dramatic chorus line which had the crowd screaming the lyrics alongside her.
She amplified the energy as fireworks exploded across Worthy Farm in Somerset to end the first day of performances at the festival.
This marks the US pop star’s second appearance at Glastonbury after making her debut in 2019 when she performed on the Other Stage.
Sam Fender took to the Pyramid Stage before Eilish to play a selection of his popular tracks, including Seventeen Going Under, Will We Talk? and Spice, to a crowd of excited fans waving flags and singing along.
After playing Getting Started, the 28-year-old singer said: “Hello Glastonbury how are we doing?
“Me and the boys have never been here before. We’ve never been here as punters, never played here before and we’re doing that all in one day, which is the most crazy experience.”
He also provided the crowd with a number of electrifying guitar solos while his band provided accompaniment.
The Pyramid Stage also hosted an array of talent on Friday, including Ziggy Marley, Robert Plant and Alison Krauss, Crowded House, Rufus Wainwright and Wolf Alice – who only just made their performance after their flight from Los Angeles was cancelled.
As acts took to stages across the festival on Friday, Sir Paul McCartney treated fans to a surprise performance at an intimate music venue in Frome, Somerset.
The former Beatle performed at the Cheese and Grain venue ahead of becoming the oldest solo headliner when he takes to the main Glastonbury stage on Saturday, exactly a week after celebrating his 80th birthday.
Earlier on Friday, Volodymyr Zelensky made an appearance via video link where he described Glastonbury as the “greatest concentration of freedom” and called for the world to “spread the truth” about Russia’s invasion of Ukraine.
He finished his speech by saying “Slava Ukraini”, “Glory to Ukraine”, which was met with loud cheers and applause at the stage in Worthy Farm, Somerset.
Other acts who performed on Friday included the Sugababes on the Avalon Stage, and Foals and St Vincent on the Other Stage, which also featured Friday performances from Kae Tempest and The Libertines.
Ukraine’s Eurovision winners Kalush Orchestra also marked their first performance in the UK since their win, with a performance on Shangri-La’s Truth Stage on Friday’s bill.
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