Talkshow queen Oprah Winfrey waved at the crowd as she arrived wearing a light pink ensemble with matching hat.
Luther star Idris Elba arrived at the same time with partner Sabrina Dhowre.
Singer Sir Elton John has also been spotted, as has British singer James Blunt, actor Tom Hardy, actress Carey Mulligan with husband Marcus Mumford and singer Joss Stone.
Oprah Winfrey waves as she arrives at St George's Chapel at Windsor Castle. Photo / AP
It was confirmed earlier this week that Sir Elton would perform at today's ceremony - 21 years after he gave a moving tribute at the funeral of Prince Harry's mother Diana, Princess of Wales.
Tennis champ Serena Williams and husband Alex Ohanian have been sighted making their way into the chapel.
British comedian and Carpool Karaoke star James Corden is among wedding guests.
The Spencer family are also represented by Charles Spencer - Diana's brother - and his wife Karen.
Idris Elba and Sabrina Dhowre arrive at St George's Chapel at Windsor Castle. Photo / AP
The Duchess of Cambridge's family, the Middletons, have also turned up. Kate Middleton's younger sister, Pippa, was all smiles alongside husband James Matthews. The couple are expecting their first child.
While Prince Harry's aunt, Sarah Ferguson, has made it to the party also.
Like no other royal wedding before it, the wedding will be sprinkled with stardust from the glittering LA-set. Friends from Meghan Markle's inner circle - including actors from her hit television show Suits - will give the wedding a touch of "LA cool".
David Beckham arrives. Photo / AP
Pippa Middleton and her husband James Matthews arrive. Photo / AP
Green smoothies as much as vintage champagne may just be the drink of choice.
Sofia Wellesley and James Blunt arrive for the wedding. Photo / AP
Tennis superstar Serena Williams and her husband were also on the guest list.
Amal Clooney and George Clooney arrive. Photo / AP
The stars of Suits, the law drama Marjkle starred in were also there, including her former on-screen boss Gabriel Macht, aka Harvey Spector.
This week, Patrick Adams, Markle's on-screen husband, posted an Instagram photo from a pub in nearby Beaconsfield, Bucks, writing: "I have historically ale'd."
