Prince Harry and Meghan Markle gazed into each other's eyes Saturday at the altar in St. George's Chapel as a soprano sang a work by Handel to celebrate their trans-Atlantic union in front of hundreds of celebrity guests and millions of viewers around the world.

The American actress arrived to a fanfare and walked down the aisle accompanied part of the way by Prince Charles, and by 10 young page boys and bridesmaids. The children included 4-year-old Prince George and 3-year-old Princess Charlotte, children of Prince William and the Duchess of Cambridge.

Markle's sleek white dress, designed by British designer Clare Waight Keller, had a boat-neck and was made of silk. Her veil came to her waist in front and billowed out for many feet behind her long train.

Prince Harry and best man Prince William wore dark formal military dress, with white gloves and the frock coat uniforms of the Blues and Royals. Harry kept his full red beard intact.

As they stood at the altar, Harry said to Meghan: "You look amazing."

Royal Wedding 2018: Clooney’s, Beckham’s, And More Celebrities Arrive At Windsor Castle Reuters

A host of celebrities and athletes — including Oprah Winfrey, Idris Elba, Elton John, George and Amal Clooney, Serena Williams, James Corden and David and Victoria Beckham — arrived in the bright sunshine at Windsor for the ceremony.

Prior to the festivities, Queen Elizabeth II honored her red-headed, 33-year-old grandson with a new title: the Duke of Sussex. That means that Markle will become the Duchess of Sussex when she says "I will."

The wedding is a global event, thanks to Harry's status as a senior British royal and Markle's celebrity after starring on the U.S. television series "Suits" for seven years. The wedding pomp and ceremony — complete with Anglican prayers and tradition, classical music, a gospel choir and a horse-drawn carriage ride through Windsor — will be beamed live to tens of millions of TV viewers across the world.

Relatives at the ceremony included Charles Spencer, the brother of Harry's late mother Princess Diana, and Harry's cousin Zara Tindall and her husband. Also in attendance was the family of Prince William's wife Kate: Parents Carole and Michael Middleton, sister Pippa Middleton and brother James Middleton.

Harry's ex-girlfriend Chelsy Davy was also among the congregation, as was Sarah Ferguson, the Duchess of York, the ex-wife of Harry's uncle Prince Andrew.

Outside the castle, thousands of fans crammed the streets of Windsor. Irene Bowdry, a lawyer from California, was aboard the jammed early train to Windsor. She booked her trip to England as soon as the wedding date was announced.

"An American in the royal family, isn't that so exciting?" she said.

The weather was balmy and clear, bathing the ancient stones of Windsor Castle in a beautiful spring light. Royal fans have been camping outside the castle for days, and British police have stepped up security, with sniffer dogs, barricades and patrols all over town.

"Windsor is absolutely bursting with excitement," said royal historian Hugo Vickers. "There are cardboard cutouts of Meghan and Harry in every shop window, virtually. There's bunting all over the place. I've never seen so many people in the streets of Windsor."

Watch: Meghan Markle Reveals Her Stunning Wedding Dress Reuters

Everyone was waiting to see which designer Markle had chosen for her wedding gown. Prince Charles, the groom's father, walked Markle to the altar after her own father became too ill to attend.

It is a time of transition for the British monarchy, one that seems filled with hope as William and Harry — the two sons of Charles and the late Princess Diana — step ever more confidently into the limelight. Many in the crowd, like Ana Karukin, a Brazilian nurse living in Florida, said they came to witness a moment of history.

"We've got to be here for him and for her, my Meghan, my special girl, to support them, because it's a beautiful time," she said. "I wish that Diana was here, but she's in heaven watching over them."

The brothers found the time amid the wedding hoopla to stroll outside the castle Friday evening to thank fans for coming. A smiling Harry gave a thumb's up and answered "Great, thank you!" when asked how he was feeling on the eve of his wedding.

And it seems to be a time of joy for 92-year-old queen and her 96-year-old husband, Prince Philip, attended the ceremony despite his recent hip replacement surgery. She saw her grandson marry a woman who clearly has brought him the happiness he's often said was missing from his life after his mother's premature death.

Preparations For The Royal Cake Are Under Way As The Royal Wedding Looms Reuters

The family tableau will play out on the broadest possible stage. Dozens of broadcasters set up on the castle grounds for a visual feast. The chapel itself is a masterpiece of the late Gothic style, and Windsor Castle has been home to British sovereigns for nearly 1,000 years.

After the ceremony, the newlyweds were set to ride a horse-drawn carriage through the streets of Windsor, flanked by a British military procession with many officers on horseback.

Two gala receptions were to follow. The first hosted in the afternoon by the queen — with finger foods, wine and champagne. The second, smaller reception is being thrown by Charles. A three-hour break in between will allow for a rest and a change of outfits for the lucky 200 invited to the evening soiree.

The chapel service reflected Markle's American roots. The Most Rev. Michael Bruce Curry, the African-American leader of the Episcopal Church, delivered a sermon, and the musical selections will included versions of Ben. E. King's "Stand By Me" and Etta James' "Amen/This Little Light of Mine."

Markle was accompanied by her mother, Doria Ragland, when she drove to the chapel, and many of her closest friends and some co-stars from "Suits" were in the audience. She opted not to have a maid of honour but there were 10 young bridesmaids and page boys, including 4-year-old Prince George and 3-year-old Princess Charlotte, the elder children of William and his wife Kate.

The couple has made an effort to honor the memory of Harry's mother Diana, whose older sister Jane Fellowes will give a reading during the wedding service.

After seeing the couple up close, Curry said Friday that he saw "two real people who are obviously in love."

"When I see them, something in my heart leaps," he said. "That's why 2 billion people are watching them."