The Duke and Duchess of Cambridge were pictured carrying their 3-month-old son into the Chapel Royal for the low-key 45 minute service this afternoon (October 23).
It is the first time that Prince George, who is dressed in a replica gown worn by Queen Victoria's eldest daughter in 1841, has been seen in public since his birth in July.
The Queen, Prince Phillip, Prince Charles, the Duchess of Cornwall and Prince Harry are among the royals known to be in attendance.
Catherine's parents Carole and Michael, as well as her siblings Pippa and James also took part in the service.
However, Prince Harry's rumoured girlfriend Cressida Bonas was not thought to be at the ceremony.
Harry and Pippa will read separate lessons, while guests will sing 'Blessed Jesu! Here we Stand', which was written for Prince William's christening in 1982. The Archbishop of Canterbury will then baptise Prince George.
The Duke's cousin Zara Phillips was confirmed as one of George's seven godparents earlier today.
Official pictures taken at Clarence House by photographer Jason Bell will be released to the public tomorrow.
Meanwhile, fans of Prince William and Catherine have taken up spots outside St James's Palace in a bid to catch a glimpse of the royals.