Bob the Builder, JK Rowling and 2,000 children were due at Buckingham Palace on Sunday to take tea with Queen Elizabeth II.
The palace gardens were being transformed into a fairy-tale playground for the party, held to mark the queen's 80th birthday and celebrate British children's literature.
Invited guests included Harry Potter creator Rowling.
The palace grounds were transformed for the day into a series of settings from children's classics, including Winnie the Pooh's Hundred Acre Wood and Thomas the Tank Engine's railway.
Models dressed as favourite literary characters including Paddington Bear, Postman Pat and supernanny Mary Poppins were due to attend, as was Black Beauty in the form of a real horse.
Guests were being treated to picnic food prepared by Naked Chef Jamie Oliver and a comic show performed by stars including actress Julie Walters and model Sophie Dahl granddaughter of much-loved children's author Roald Dahl.
The children, aged between 4 and 14, won tickets to the party in a lottery.
The queen turned 80 on April 21, although her official birthday, celebrated with the annual Trooping of the Color parade, is marked on June 17.