After reaching the Abbey, the men walked down a red carpet into St. Edmund's Chapel, mingling with the wedding guests. Harry stopped and talked to the Spencer family, Princess Diana's family.
Male guests, including singer Elton John and soccer star David Beckham, sported elegant suits with tails or military uniforms while the ladies arrived wearing a riotous collection of elaborate hats. The hats are a fashion accessory ranging from giant brims in bright colors of pink, purple and yellow to tiny hats shaped like flowers perched on their foreheads. Others have sprigs and feathers sprouting from them. Mobile phones and cameras were banned at the wedding.
"The whole of the British upper lip kind of unbuttons with these hats," said Tina Brown, editor of Newsweek and the Daily Beast.
As the couple exited the abbey following their wedding, the bride curtsied to Queen Elizabeth and William bowed to his grandmother.
Hours after the ceremony and balcony appearance, the newlyweds surprised their fans by driving out of Buckingham Place in a convertible festooned with ribbons, trailing a tail of balloons and bearing the license plate "Ju5t Wed." Kate was still in her wedding gown as they drove to St. James Palace to rest before the evening party began.
Along with becoming a duke, William also became Earl of Strathearn and Baron Carrickfergus, which means Middleton will become the Countess of Strathearn and Baroness Carrickfergus.
Carrickfergus is in the Northern Ireland county of Antim and has its own castle. Strathearn is in Scotland. Since William is also the Prince of Wales, his titles now encompass all the elements of Great Britain: England, Scotland, Wales and Northern Ireland.
Princes William and Harry spent Thursday night with their father, Prince Charles, having a private dinner. Queen Elizabeth greeted the foreign royals attending the wedding last night at the Mandarin Oriental Hotel in Hyde Park.
Scotland Yard beefed up security for the wedding, using 5,000 policemen -- both in uniform and undercover -- as close to the new royal couple as at the back of their carriage.
Scotland Yard officials said earlier this week that they were ready for anything that might disrupt the wedding.
"Any criminals attempting to disrupt it, be that in the guise of protest or otherwise, will be met by a robust, decisive, flexible and proportionate policing response," the London police officials said.
More than 1,500 soldiers, sailors and air crew are on duty to line the couple's procession route between Westminster Abbey and Buckingham Palace, just under a mile away. The couple plans to travel after the wedding from Westminster Abbey to Buckingham Palace in an open topped carriage, the 1902 State Landau.
ABC News' Linda Albin, Michael S. James, Aaron Katersky, Gabriel O'Rorke and Jean-Nicholas Fievet contributed to this report.
Watch a special 2-hour edition of "20/20" at 9 p.m. ET tonight to look at the big day, the night-time celebrations, and what's next for the happy couple, with Barbara Walters.