But the weather seems unlikely to co-operate with the meticulously choreographed, carefully rehearsed event that will be attended by kings and queens, politicians and priests, and celebrities ranging from Elton John to David Beckham.

Showers are predicted, a possible dampener on street parties being held across the United Kingdom, as well as protests threatened by Muslim radicals, anarchists and anti-royalists.

About 5,000 police officers will be on the streets, including 110 on horseback and 35 with sniffer dogs -- but the world's media have deployed even more resources, with at least 8,000 staff working in London for the event.

British Culture Secretary Jeremy Hunt has predicted the ceremony will be seen by an estimated two billion people worldwide.

After the ceremony the newlyweds will be taken by carriage from the abbey to Buckingham Palace, cheered on by thousands of wellwishers. Later the couple will kiss on the balcony of the palace in front of the crowds while Royal Air Force jets fly overhead to mark the celebrations.

William announced his engagement to Middleton in November after proposing secretly in a rustic cabin in Kenya in October.

He's the older son of Prince Charles and the late Diana, princess of Wales; the most famous grandson of Queen Elizabeth II; a military search-and-rescue helicopter pilot and, by one recent measure, the most popular member of the royal family.

She's an art history graduate and the eldest child of self-made millionaires who run a party-supplies company.

They met as college students at the University of St. Andrews in Scotland, sharing an apartment with a circle of friends before they began dating. She first caught his eye when she modelled in a student fashion show wearing a see-through dress over her underwear, British reports have said -- though there's no chance she'll wear anything similar when her much-anticipated wedding dress, the last major secret element of the wedding, is finally revealed.

The couple will be married in front of nearly 2,000 guests at Westminster Abbey, where British monarchs are crowned, sometimes married, and often buried, alongside major figures from British history including Charles Darwin, Charles Dickens, Isaac Newton and George Frederic Handel.