Louis' baptism comes after it was recently revealed he will have no less than six godparents, who are all friends or family of Their Royal Highnesses.
Queen Elizabeth II and Prince Philip couldn't make it to the event.
Prince Louis' first godmother is Lady Laura Meade.
The Duchess of Sussex stepped out with the Duke of Sussex in a black scoop-neck dress, black stiletto heels and a black clutch bag.
The woman of the hour, Catherine, Duchess of Sussex, continued her tradition of wearing cream to all her children's christenings in Alexander McQueen.
The service for Prince Louis was attended by guests including Prince Charles and his wife Camilla, and newlyweds Prince Harry and Meghan.
Archbishop of Canterbury Justin Welby, the spiritual leader of the world's Anglicans, will lead the service in the Chapel Royal at St James's Palace.
And little Charlotte couldn't resist sticking her tongue out at crowds while stood alongside her brother - who also pulled a face or two.
After the ceremony, the guests will descend on a private tea afternoon at Clarence House and indulge in some desserts - including a tier from Kate and William's 2011 wedding cake.
Prince George and his younger sister, Princess Charlotte, were snapped at Buckingham Palace watching a Royal Air Force flypast from a window.
Harry Aubrey-Fletcher: He went to Ludgrove School and Eton with William and his father is the Lord Lieutenant of Buckinghamshire. Carter is a college friend of Kate's, and she and her husband Robert live in the county of Norfolk near Anmer Hall, William and Kate's country home.
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