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

Sources told ABC News royal contributor Omid Scobie that Prince Harry, Duke of Sussex, will be heading back to the United States soon after attending his father's coronation in London on Saturday.

"I'm here on the Duke of York steps, we've just seen the procession go back to Buckingham Palace and for Prince Harry, the Duke of Sussex, that probably brings his very quick trip to London to an end," Scobie said. "We saw him arrive on Friday evening getting ready, of course, for the big day to support his father at the coronation ceremony."

"But, of course, he also has duties back in Montecito, California, and sources close to the duke say that it is going to be an in-and-out trip for him," he added. "Just here to be part of that moment in Westminster Abbey, support his father and then be back at home to support his own family in Montecito."

The newly-crowned king and queen received a royal salute upon their return to Buckingham Palace.

Members of the military gave three cheers and played the national anthem as Charles and Camilla, wearing their crowns and robes, looked on.

As the new king and queen passed by in a golden carriage, hundreds of anti-monarchy protesters booed loudly in Trafalgar Square.

They also yelled chants of "what a waste of money" and "not my king."

-ABC News' Patrick Reevell

Royal watchers viewing the coronation will notice three members of Charles’ family missing from today’s festivities.

Harry’s wife, Meghan, the Duchess of Sussex, and their two children, Prince Archie and Princess Lilibet, did not attend the coronation.

Instead, they are home in California, where the family now lives full-time.

Today is Archie’s fourth birthday and Harry is expected to leave shortly after the coronation service to return home to his family.

As a non-working royal, Harry, who sat near his cousins Beatrice and Eugenie inside Westminster Abbey, did not join other members of the royal family in the procession back to Buckingham Palace.