Hundreds of millions of people across the world are expected to tune into watch the wedding of Queen Elizabeth's popular grandson and the American star of the TV drama Suits at the 15th century St George's Chapel in Windsor Castle.
As dawn broke over its ancient stone walls, excited crowds began to assemble, many draped in British and American flags.
"I just think it will be magical. There will be so much shouting and cheering and noise. I love the royal family," said Ms Ronda Musgrove, 59, who camped overnight on the street opposite.
More than 100,000 fans will cram the narrow streets of the town 30km west of London, dominated by the oldest and largest inhabited castle in the world, a home to 39 English monarchs since 1066.
"I love all the regalia, all the hoo-hahs, the procession with the cavalry," said Ms Jane Tofolo, 50, from south-west London, who shook hands with Prince Harry when he held an impromptu walkabout with his brother and best man outside the castle on Friday evening.
"It will be spectacular because us Brits do all that very well."
Prince Harry, 33, is sixth in line to the British throne. Ms Markle, 36, is a divorcee whose mother is African-American and father is white.
For many, the wedding is a fairytale. For some black Britons, it illustrates the breakdown of barriers in modern Britain.
To others, it is an irrelevance. Polls have suggested that most Britons will not bother tuning in to watch the event.
The hour-long ceremony begins at 1100GMT, with weather forecasts predicting blue skies and sunshine.
The bride will arrive at the church with her mother, Ms Doria Ragland, 61, with whom she was spending Friday night at a luxury hotel.
Harry was staying at another hotel with elder brother and best man Prince William, whose daughter Charlotte and son George will be among the bridesmaids and page boys.
Prince Harry's father and heir-to-the-throne Prince Charles will walk his future daughter-in-law down the aisle after Ms Markle's father pulled out due to ill health.
Mr Thomas Markle, 73, a lighting director for TV soaps and sitcoms, told the US celebrity website TMZ he had undergone heart surgery on Wednesday.
Confusion over his attendance marred the build-up to the wedding, which had been carefully choreographed for months by royal aides.
It will also include a smattering of celebrities, including fellow cast members of Ms Markle from Suits.
The service will be conducted by the Dean of Windsor, with Justin Welby, the Archbishop of Canterbury and the spiritual head of the Anglican Church, overseeing the exchange of vows.
A black US bishop, Most Reverend Michael Bruce Curry, will give the wedding address.
In a sign of modernity for the royals, Prince Harry will wear a ring while Ms Markle will not vow to obey her husband. Senior male British royals do not traditionally wear rings.
Interspersed with traditional hymns, a choir will perform American soul singer Ben E. King's 196o's hit Stand by Me.
After the ceremony, the newlyweds are expected to greet some of the 1,200 members of the public specially invited into the grounds of the castle before setting off on a carriage procession through Windsor.
A reception will be held in the castle's St George's Hall before 200 guests join the couple at an evening event at Frogmore House, another grand mansion in the grounds.
The prince and his new wife, who are expected to be given a new title by the Queen to mark their marriage, are not immediately leaving on honeymoon and will carry out their first official engagement as husband and wife next week.
Educated at the exclusive Eton College, a stone's throw from Windsor, Prince Harry, the younger son of the late Princess Diana, gained a reputation as a royal wild child.
He admitted smoking cannabis, getting drunk when underage in a pub, scuffled with paparazzi outside a London nightclub and drew outrage by dressing as a Nazi officer at a party.
But he turned his image around after joining the army, where he spent 10 years and included two tours of duty in Afghanistan.
Last year, he spoke openly of his emotional torment following the 1997 death of his mother in a Paris car crash. Aged just 12, he walked solemnly behind her coffin in the funeral cortege.
He met Ms Markle on a blind date in July 2016 set up through a mutual friend. Prince Harry said he had never heard of his future wife or watched her TV series, and she said she knew nothing of the prince.
But after just two dates, he whisked her off to Botswana for a holiday, camping under the stars.
"The fact that I fell in love with Meghan so incredibly quickly was confirmation to me that all the stars were aligned, everything was just perfect," Prince Harry said when their engagement was announced last November.
Prince Harry's gilded upbringing is in stark contrast with Ms Markle's. She was born and raised in Los Angeles and her parents divorced when she was six.
After a number of minor roles in films and on TV, she won the role as Rachel Zane in Suits.
She ran a successful lifestyle blog, thetig.com, and has worked as a humanitarian campaigner. In 2011, she married film producer Trevor Engelson, but they divorced in 2013.
The intense spotlight on her father this week was indicative of the limelight she now faces.
Prince Harry issued a public rebuke to journalists when they were dating, accusing some of sexism and racism.
The prince has long had a testy relationship with the media. His mother's limousine was being chased by paparazzi just before her fatal crash.
Over the last 20 years since Diana's death, the royals have enjoyed much better media coverage with a sophisticated PR operation helping to turn round the perception of a hopelessly out-of-touch institution where the tribulations of the Queen's children were played out in national newspapers.
Queen Elizabeth famously described 1992 as an "annus horribilis" (horrible year) after three of her four children's marriages failed, including Prince Charles' marriage to Princess Diana, and part of Windsor Castle was badly damaged by fire.
The British remain broadly supportive of the monarchy albeit with a sense of mild irony about the pomp and pageantry that accompanies it, though many have deep respect for Queen Elizabeth, after her 66 years of service.