Prince William and his best man and brother Prince Harry have arrived at Westminster Abbey for his wedding to Kate Middleton.
Some 1,900 people are invited to the church service, with the ceremony getting under way at 1100 BST.
Thousands of people have camped out overnight on the procession route the couple will take to Buckingham Palace after the service.
The couple are to use an open-topped carriage for the procession.
Despite the risk of showers, the pair will travel in an 1902 State Landau carriage, which was used by Prince William parent's for their wedding in 1981.
The day has started off dry and cloudy in London but the Met Office says there is a risk of heavy showers developing later on. Temperatures in the capital are expected to reach a high of 19C (66F) in the afternoon.
The prince is wearing the red tunic of an Irish Guards colonel - his most senior honorary appointment - but the bride's dress design is being kept secret.
Prince William has been given the title of the Duke of Cambridge by the Queen, and Miss Middleton will become Her Royal Highness the Duchess of Cambridge on their marriage.
The prince has also became the Earl of Strathearn and Baron
