Prince William was joined by 300 guests for an African-themed 21st birthday party at Windsor Castle.
The Windsor streets had a jungle flavour as guests arrived
All 300 people at Saturday evening's celebrations - including the Queen and other members of the Royal Family - wore fancy dress.
An intruder was arrested inside the castle during the party and taken to Thames Valley police station for questioning.
No one was injured during the incident at 2320 BST, and thorough review into security is under way.
As the party started, outfits spotted arriving at the castle included a furry lion, Tarzan and a banana.
The castle's ancient rooms were transformed into scenes from the African bush, which include a life-sized elephant made out of papier mache.
African band Shakarimba had been flown in specially from Botswana where William heard them perform while on holiday four years ago.
The party theme - Out of Africa - was inspired by the prince's time in Africa during his gap year before university.
BBC correspondent Clarence Mitchell said the event was strictly for family and friends only, to ensure the prince could fully relax.
Among the guests were Camilla Parker Bowles, Earl Spencer and his sisters, and William's friends from St Andrews University in Scotland.
The earl arrived dressed in a light-coloured safari type suit, followed by the comedian Rowan Atkinson and his wife in linen and colonial-style attire.
Our correspondent said the party was the most significant meeting between the Windsor and Spencer families since the earl's eulogy at Princess Diana's funeral, which was critical of the Royal Family.
Other outfits seen on arriving guests included a lion suit topped with a gold crown, a full foreign legion uniform, a Biggles-esque pilot, a banana and a top-hatted witch-doctor.
The Prince of Wales helped organise the party.
In the first part of an interview given to mark his birthday, Prince William rounded on his father's critics.
The prince, who was interviewed at St James's Palace by the Press Association, also posed for photographs by Mario Testino - the renowned celebrity photographer who famously took pictures of his mother Diana.
300 guests were invited to the event
He summed up his own personal goals as he embarks on adulthood as being "honest, genuine, thoughtful and caring".
And in the second part of the interview published on Sunday, he denied rumours he did not want to be king, saying it was his duty.
The prince is midway through a four-year degree course at St Andrews, studying history of art.
The band playing at the party, Shakarimba, are a marimba band from a remote village, and William joined in during their rehearsals at Windsor Castle.
The six-piece band were encouraged and nurtured by Steve Harpt, the American headmaster of their small bush school.
The band members made their own instruments from mukwa wood.
Mr Harpt has moved on to set up a primary school for orphans and destitute children, to which William is giving some of the proceeds from the sale of Mario Testino's photographs.
As he turns 21, the young prince steps into the role of Counsellor of State with delegated powers to act on behalf of the Queen.
In this he joins his father, the Prince of Wales, the Duke of Edinburgh, the Duke of York and the Earl of Wessex and replaces the Princess Royal.