Middleton at Westminster Abbey.
The first of the 1,900 guests will arrive from 0815 BST, with the ceremony itself getting under way at 1100.
Thousands of people have camped out overnight on the procession route the couple will take to Buckingham Palace.
William will wear the red tunic of an Irish Guards colonel - his most senior honorary appointment - but the bride's dress design is being kept secret.
The Met Office says the day will start off dry but cloudy in London but there is a risk of heavy showers developing later on.
On Thursday night, Prince William went on an impromptu walkabout to meet royal watchers who had thronged to The Mall, near the palace.
He spent several minutes shaking hands, chatting and posing for pictures telling well-wishers: "All I've got to do is get the lines right."
William spent Thursday evening with the Prince of Wales, Duchess of Cornwall and Prince Harry, while Miss Middleton and her family gathered at the Goring Hotel, in Belgravia, a short distance from Westminster Abbey.
BBC reporters at the scene estimated between 3,000-5,000 people had been camping overnight in The Mall and around Westminster Abbey, with more than 600,000 expected to line the streets by the time of the service. About two square miles of central London have been closed to traffic.
Scotland Yard says 5,000 officers will be on duty, with more than 900 along the wedding route. Most will be in uniform, but some will be in plain clothes, mixing with the crowd.
