Crowds are gathering in central London for the wedding of Prince William and Kate 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.
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 3,00-5,000 people had been camping overnight in The Mall and around Wewstminster 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.
Elsewhere in the UK more than 5,500 street parties are taking place – including one hosted by Prime Minister David Cameron at Downing Street and another by the anti-monarchy campaign group Republic.
After the service, five carriages drawn by mounted troops of the Household Calvary will carry the newlyweds, the Royal Family and the Middleton family on the 15-minute journey from Westminster Abbey to Buckingham Palace.
The route will take them past the Houses of Parliament and Downing Street and, under rows of Union Jacks, along The Mall, to Buckingham Palace, where the couple will appear on the balcony at about 1325 BST.
Royal officials said William and Kate have been involved in planning their wedding day, from the music at the ceremony to the flowers and the cake.
Inside Westminster Abbey itself, six field maples and two hornbeams will line the aisle leading up to the altar.
Some 650 guests have been invited to the palace for a buffet lunch and about 300 close friends and relatives will stay on for formal black tie dinner and disco in evening.
But the Queen and the Duke of Edinburgh will leave for a weekend away after hosting the lunchtime reception and will miss Prince Harry’s best man speech and Michael Middleton’s father of the bride address.
The Archbishop of Canterbury will conduct the wedding ceremony, which will see Miss Middleton vow to “love, comfort, honour and keep” Prince William but not to obey him.
