Prince Harry will marry his 36-year-old American fiancée, actress Meghan Markle, Saturday, with the world watching.

The highly-anticipated royal wedding could become the most viewed event in history, with an estimated 3 billion people expected to watch.

Since Harry and Meghan are getting married within the smaller confines of Windsor Castle (instead of Westminster Abbey in central London), the couple sought to arrange their wedding so that as many people as possible could share the experience in person.

The couple have invited 2,640 people to watch them and their guests arrive at and depart from St. George’s Chapel, including 1,200 people nominated by authorities in nine of the United Kingdom’s regions, as well as charity workers, local school children, and employees of the queen. Six hundred guests will attend the ceremony inside the Chapel.

Here's the news breakdown from Windsor:

6 a.m. London time/1 a.m. EST: The first train from central London to Windsor left at 5 a.m. local time. It was packed full for the 45-minute ride.

Several hundred tourists, some with small children in tow wiping the sleep out their eyes, were in an excited, expectant mood for Harry and Meghan's big day.

About halfway to Windsor, a cry rang out from a voice in one of the carriages.

“Are we excited!?”

“Yes! Yes! Yes!” a group of women from Florida shouted back, before erupting into giddy laughter.

James Turner, a 33-year-old from southern England who was seated near USA TODAY, asked: “Do Americans always get this worked up about weddings?”

He said he too was on his way to Windsor, but if not for his girlfriend’s insistence he would not have made the effort to get up so early to avoid the crowds. More than 100,000 people are expected in Windsor on Saturday.

By 6 a.m. tens of thousands were already on the streets cheering, waving flags and ready to party in Harry-and-Meghan royal memorabilia from earrings to scarves.

Earlier Saturday, Carol Ann Duffy, Britain’s poet laureate, published an ode to commemorate the royal wedding. It read:

“It should be private, the long walk on bereavement’s hard stones; and when people wave, their hands should not be mobile phones, nor their faces lenses; so your heart dressed in its uniform.

On. Then one blessed step and the long walk ended where love had always been aimed, her arrows of sweet flowers gifting the air among bells — yes, they all looked —and saying your name.”

7:45 a.m. London time/2:45 a.m. EST:

Londoner Grace Gothard was wrapped in the flag of Ghana, where she originally hails from.

Gothard was wondering around the Long Walk, the three-mile, tree-lined avenue that the royal couple will ride down in an open-top, horse-drawn carriage after they are married in St. George’s Chapel in Windsor Castle.

Gothard, accompanied by her husband Mike, had some advice for Markle: “I want you to be yourself after you are married, Meghan. Be a great woman. Be a great human being. And be happy.”

8:11 a.m. London time/3:11 a.m. EST:

Buckingham Palace announced the royal tiles bestowed by Queen Elizabeth for Harry and Meghan — the Duke and Duchess of Sussex.

“The Queen has today been pleased to confer a Dukedom on Prince Henry of Wales. His titles will be Duke of Sussex, Earl of Dumbarton and Baron Kilkeel,” the palace said in a statement. “Prince Harry thus becomes His Royal Highness The Duke of Sussex, and Ms. Meghan Markle on marriage will become Her Royal Highness The Duchess of Sussex.”

9:00 a.m. London time/4 a.m. EST:

Spectators heading into Windsor town center hoping for a view of the festivities were diverted by specially-appointed marshals down the winding streets with many closed.

Hawkers were selling memorabilia including royal wedding flags and scarves. Large groups of police officers were stationed on the banks of the River Thames, and close to the castle.

There was a carnival atmosphere opposite the castle, as the excited crowd waited for the day’s events to unfold.

In nearby Eton, a sign announced a road leading to the bridge to Windsor would be closed for a street party: one of many being held to mark the big day. Some here did’t have the day off to watch the festivities. Boys from the exclusive Eton College — Prince Harry’s old school — headed for lessons as usual on Saturday.

Contributing: Kim Hjelmgaard and Jane Onyanga-Omara in Windsor, England