The 20-year-old singer’s comments come after 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, the singer spoke out against 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 of adoring fans 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 delivered another political moment during the set as she spoke of 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.”
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.”
The singer closed her history-making show with an explosive rendition of her hit track Happier Than Ever.
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.
Other artists who played. the festival on the. first full day included Sam Fender, Ziggy Marley, Robert Plant and Alison Krauss, Crowded House, Rufus Wainwright, Sugababes, Foals, St Vincent, The Libertines 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 Zelenskyy 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.
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.