King Charles III's coronation: The biggest moments of the historic event

Helping to carry Charles’ robe down the aisle at Westminster Abbey is his oldest grandson, 9-year-old George, the second in line to the throne.

"This is a really big moment for Prince George," said ABC News royal contributor Victoria Murphy. "All eyes will be on him because he is there as a future king. He’s not just there supporting his grandfather. He’s there seeing before him what, unless things change, one day he could be at the center of."

Joining George as his fellow pages of honour for Charles are Lord Oliver Cholmondeley, Master Nicholas Barclay and Master Ralph Tollemache.

Princess Anne, Charles' only sister, has joined royal family members at Westminster Abbey to watch her brother's coronation.

Anne and Charles were both alive when their mother was coronated in 1953. While Charles attended the ceremony, Anne did not because she was considered too young at the time.

Anne will play a special role in Charles' coronation, leading the procession back to Buckingham Palace following the service at Westminster Abbey.

Prince Harry has arrived at Westminster Abbey, marking the first time he has appeared at a royal event since last September, when he attended the queen’s funeral.

Harry, the fifth in line to the throne, is not expected to play a role in the coronation service.

He is attending alone, without his wife Meghan, or their children, Archie and Lilibet, and is expected to depart home to California shortly after the service.

As a non-working member of the royal family, Harry is dressed in a morning suit.

He entered Westminster Abbey with his cousins, Princesses Beatrice and Eugenie.

Though every coronation is rich in tradition, Charles’ coronation will look very different from his mother’s 70 years ago.

On Saturday, 6,000 fewer people will fill the pews of Westminster Abbey for today's coronation than they did for his mother's, but it will be a much more diverse and inclusive crowd.

“This is an ancient ceremony that goes back 1,000 years, but [the royals] want to use it to showcase modern Britain today,” said ABC News royal contributor Victoria Murphy. “So we're going to see inside the Abbey, ordinary members of the public in the seats that would have been occupied by aristocrats 70 years ago, and also a lot of diversity.”

“We're going to see lots of women, which was not the case in 1953, people of different ethnicities, different religions,” she said. “They’re really putting front and center the diversity that exists in Britain today.”