Forecasters predicted a 70 percent chance of rain showers early Friday diminishing or ending by the time the service is to begin at 11 a.m. London time (6 a.m. EDT).
Temperatures will be in the low to mid-60s. Winds will be from the east around 14 mph.
William, 28, in a charcoal-gray suit, and Middleton, 29, in a black dress, went through a full and final wedding rehearsal at Westminster Abbey Wednesday night. They were accompanied by William's brother, best man Prince Harry, 26, Middleton's sister, maid of honor Pippa Middleton, 27, and the Middleton parents, The Telegraph and People magazine reported.
Also present were Archbishop of Canterbury Rowan Williams, who will marry the couple, Bishop of London the Right Rev. Richard Chartres, who will give the address, and Dean of Westminster the Very Rev. John Hall, who will conduct the service.
The couple's chauffeur-driven black police Range Rover, flanked by a police escort, arrived at 7.30 p.m. for the run-through. The couple left just after 9 p.m.
"In America, the crowd would have been screaming. But here, everyone was very quiet and orderly," Carolyn Houghton of Medford, Ore., told People. "But I do wish they would have stopped and waved for a minute."
Thursday night William plans to have a private dinner with his immediate family, Prince Charles, Camilla and Harry, at Clarence House, their official London home, situated on The Mall near Buckingham Palace, St. James's Palace said.
Middleton and her family will have their own private meal at the luxury, Edwardian-style Goring Hotel, also near Buckingham Palace.
The Middletons have rented a five-bedroom, $2,550-a-night suite. Queen Victoria's framed silk bridal gown and tiara from 1840, usually displayed in the suite's wardrobe, were removed "for fear it will give Kate pre-wedding jitters," The Daily Mail said.
William's grandparents, Queen Elizabeth and Prince Philip, will have a dinner for wedding guests from foreign royal families at the 110-year-old Mandarin Oriental Hyde Park in London's exclusive Knightsbridge district.
Charles and Camilla will make an appearance for pre-dinner drinks before slipping out for dinner with their sons.